308 C. Spence Bate — On Archcsasfacus in the Lias. 



While studying the fossil forms of the Eryonidce, for comparison 

 with those recently brought to our knowledge through the deep-sea 

 explorations, I have found in the collection of Mr. J. E. Lee, F.G.S., 

 of Torquay, a specimen from the Lower Lias at Lyme Eegis, that 

 appears to connect the ancient forms with the recent more intimately 

 than has been illustrated in the comparison of any previously known 

 fossil specimen. 



The specimen is in a fragile and imperfect condition, but one-half 

 of the dorsal surface is tolerably well preserved, while the other half 

 exhibits the impress of the form only. The two conditions are 

 shown in the accompanying figure by a different depth of shading, 

 the more dark by that where the external texture is preserved ; the 

 less where the impression of form alone is retained ; while the outline 

 exhibits the restoration of structure in conformation with known 

 parts. 



Arcli^astacus, the generic name by which I propose to call the 

 present genus, has the dorsal surface of the carapace almost circular, 

 the anterior margin being nearly straight between the orbital notches, 

 while beyond them the anterior lateral angles are produced anterioi'ly 

 beyond the frontal margin. The anterior two-thirds of the lateral 

 margin are smooth, whereas the posterior third is armed with five 

 prominent teeth. The median dorsal line is longitudinally armed 

 with three or four strong teeth, one not being distinct, on the frontal 

 margin a second almost hypothetically present over the gastric 

 region, a third and fourth over the post-gastric and cardiac regions, 

 and evidence exists of a double row of small tubercles traversing 

 the dorsal median line from the posterior to probably the frontal 

 margin. The inner line of the branchial region is posteriorly defined 

 by a small ridge that is furnished with three or four small tubercles 

 or teeth. From the post-gastric region to the lateral walls a strong 

 ridge traverses the line of the cervical fossa in recent Crustacea, a 

 circumstance that I believe to be due to the compression of fossiliza- 

 tion, the weaker parts yielding, while the stronger and more rigid 

 resist. Thus the fossa, which is due to a reflexion or folding of the 

 dermal tissue, resists more decidedly the compression of fossilization, 

 and thus remains rigid while the surrounding tissue has yielded to 

 pressure. The cervical fossa, or as it may in this specimen be called 

 ridge, bifurcates at half its length, forming an anterior and posterior 

 branch, which incloses what I have in recent species called the 

 Siagnitic region, or that part to which the Siagnos , or mandible is 

 attached. The posterior portion of the animal, the pleon (or 

 abdomen), is broad and evenly tuberculated ; each somite generally 

 carrying (or supposed to carry) one large tubercle on the posterior 

 margin in the median line, a similar one near the margin, centrally 

 situated above the coxal plate, and another between this and that 

 on the median line, but of smaller dimensions, and standing on the 

 posterior margin. 



The animal appears to have no ophthalmopod, or eye-stalk, although 

 a semicircular notch appears to exist. This may arise, as I believe 

 it does, from the organ having, from its softer condition, perished 



