6 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Survey with that of a Cancer irroratus about four inches wide, notable 

 differences are to be seen. Though the four ridges are the same in length, 

 width apart, and in general arrangement, the teeth on these ridges are 

 in C. proavitus very much larger, dnd are represented in C. irroratus 

 by more numerous and crowded granulations, which are flattened, 

 rounded, and polished. 



In some important respects the tertiary species resembles G. borealis. 

 This differs from C irroratus in being finely muricate, the minute 

 setiferous tubercles being much larger, higher, and sharper, almost form- 

 ing sharp spines ; they are also more numerous and crowded, and often 

 bear a hair. In these respects G. proavitus approaches C. borealis. As 

 in G. irroratus, the antero-lateral margins are nine-toothed ; of these 

 teeth the ninth, or that next to the orbit, is sharper than in the other 

 living species, and ends in a sharp spine, with several accessory spinules. 

 The postero-lateral margins are more sinuous than in G. irroratus, and 

 the granulations on the ridge are larger, fewer, and end in a point. In 

 both of the living species the convexity of the carapace is about the 

 same. The surface in G. borealis is perhaps a little more uneven. 



The hand of G. borealis differs from that of G. irroratus in being 

 much more muricate or spiny, the granulations on the four external 

 ridt^es of the latter being represented by well-marked sharp spines, these 

 being especially large and high on the uppermost ridges. 



G. borealis is a decidedly hairy species, whereas C. irroratus is 

 naked, but a few hairs being visible ; on the other hand, in G. borealis 

 nearly every tubercle bears a pale hair. 



The abdomen of G. borealis differs from that of the more common 

 species (C. irroratus) in being less acute and mucronate at the tip. 

 The sternum is a little more hairy. 



Lencrth of a small G. borealis, 30 mm. ; breadth, 39 mm. ; thickness 



of body, 10-11 mm. 



The phylogeny of the Eastern American species of the genus 

 Cancer. — A comparison of the miocene tertiary species of Cancer with 

 the two species now living in the waters of Vineyard Sound, brings out 

 the interesting fact that the extinct species appears to be the stem or 

 ancestral form from which the recent species mentioned have descended. 



Gancer proavit^cs presents characters in which it resembles G. bore- 

 alis as well as G. irroratus. It resembles C. borealis in the higher, 

 more pointed granulations on the postero-lateral margin of the carapace, 

 and in the quite high and sharp spines on the ridges of the hand, as well 

 as the numerous setiferous spines and hairs ; on the other hand it is simi- 



