JOURNAL OF PALEONTOLOGY, VOL. 14, No. 5, PP. 519-520, 6 TEXT FIGS., SEPTEMBER, 1940

A PLEISTOCENE OCCURRENCE OF LIBINIA DUBIA, A BRACHYURAN

WILLIAM H. EASTON Walker Museum, The University of Chicago, Chicago, 111.

N THE summer of 1938 an unusual speci- length and 24.5 mm. in width, including I men of the spider crab, Libinia dubia spines, so the animal at death was probably Milne Edwards, was collected from the about one-third grown. By reason of sub- Pamlico formation (late Pleistocene) at sequent accidental breaking of the cephalo- Wailes Bluff on the Potomac River near thorax, the inner part of the animal was Point Lookout, Maryland. The fossil was revealed. As the thoracic walls are almost

FIGS. 1-6.—Libinia dubia Milne Edwards. 1, Ventral view of sternum, XI; 2, surface of digestive gland shown at right center of 5; X10; J, dorsal view of carapace, XI; 4, left side of sternum with thoracic wall above the five openings for leg muscles, X1; 5, dorsal view of sternum, looking in the thoracic cavity, X1; 6} ventral view of interior of carapace with digestive gland adhering to the carapace XI. removed from a layer one foot below the complete, details of the partially calcified top of the so-called "bed below the oyster chitinous interior skeleton can be studied. bed." Moreover, within, and occupying about The specimen consists of the cephalo- half of the thoracic cavity, is a brown thorax and the sternum, the abdomen and nodosely tubular structure, which, when appendages (except a few fragments of the carefully examined, appears to be no en- pleopods or swimmerets adhering to the crusting organism, but part of the viscera sternum) not being found. The individual of the spider crab itself. Because of its size, was a female crab measuring 28.0 mm. in its manner of filling the thoracic cavity, its

519 520 WILLIAM H. EASTON

vesicular structure, its color, and its caeca- Modern representatives of L. dubia are like appearance, it is concluded that this distributed along the Atlantic coast from chitinous material was within the hepato- Cape Cod, Massachusetts to Corpus Christi, pancreas of the crab. Thus, although the Texas, and also along Cuban and West remains of Callinectes so commonly found African strands. This crab is found on mud in the same deposit are fragmental and flats, oyster bars, and as deep as 25 fathoms poorly preserved, this representative of in varying bottom habitats. Libinia dubia Libinia, apparently by reason of better cal- is one of nine modern species of the genus cification of its chitinoid skeleton, is so but is not the most common spider crab remarkably well preserved as to retain part today. As a fossil, it has been reported of its digestive gland. previously only in the form of isolated claws. The openings in the thorax for the muscles One right finger is known from the York- of the five large appendages are well shown, town formation (upper Miocene) of Vir- as are also at least three other much smaller ginia and eight fingers were recovered from openings for the muscles of the mouth ap- the Cape May formation of New Jersey pendages. These three openings succes- (Rathbun, 1925, 1935). The latter deposit is sively decrease in size anteriorly as tKe size approximately the same age as the Pamlico of the muscles decreases. Likewise, the formation. area of cross section of the gill chamber The author wishes to thank Miss Phoebe (used as an index of volume) is restricted Jane Beall, who discovered this specimen anteriorly, resulting in the presence of either (Walker Museum no. 45193) and presented smaller or fewer gills associated with the it to him for study. appendages of the mouth. From this de- cided reduction in gill size it might be REFERENCES postulated that Libinia dubia is evolving RATHBUN, M. J., 1925, The spider crabs of toward the decapod crustaceans with five America: U. S. Nat. Mus., Bull. 129, pp. 313, pairs of gills, and that most of the transi- 318-321. , 1935, Fossil Crustacea of the Atlantic and tion had already been achieved by the Gulf Coastal Plain: Geol. Soc. America, Special Pleistocene epoch. Paper 2, pp. 113, 119.

MANUSCRIPT RECEIVED BY THE EDITOR JANUARY 11, 1940.