The Fossil Crab of Gay Head. 587 



segment on each side, where it passes beneath the abdo- 

 men, we find the genital tubercle, as represented in fig. 4. 

 These tubercles have thus the same situation as in Portu- 

 nids, while in the crabs most nearly allied to that here 

 described, it is more frequently found near the anterior 

 margin of the segment. The last or posterior segment of 

 the sternum does not project laterally beyond the margin 

 of the male abdomen, as is usual in Ocypodoids, but is 

 much narrower than the preceding segment, and entirely 

 concealed, as in Cancroids. 



The abdomen of the male only has occurred (PI. XII., 

 fig. 3). It is quite broad for a male abdomen, and its 

 middle three segments are not soldered together, but dis- 

 tinct. I have seen no specimens in which its basal joints 

 are well preserved, but they cannot differ much from the 

 representation given of them in the diagram. 



The male organs, or verges, in this genus arise directly 

 from the coxal joints of the posterior feet, and not from the 

 sternum. The posterior segment of the sternum is not 

 distinctly grooved for their passage to the intromittent or- 

 gans, although there is a slight depression ; but this is very 

 little deeper than in Carcimts, with which genus ours shows 

 much resemblance in the structure of this part of the body. 



As illustrations of this fossil crab, I have given my work- 

 ing diagrams, instead of figures drawn from nature ; for 

 to represent all the parts in the diagrams by the latter 

 method, the delineation of an inconveniently large number 

 of specimens would be required. 



It will be seen from the foregoing description that Arch- 

 aeoplax signifera presents, in its different parts, the charac- 

 ters of widely-removed groups of Brachyura. This indeed 

 is the case with most of the genera of the family Carciiio- 

 placidae, in which I have placed it. In its carapax and 

 elongated orbits we find most resemblance to Ocypodoids, 

 for though certain Cancroids, like Trapezia, have a quadri- 



