1898.] POST-LABVAL STAGES OF THE COMMON CRAB. 207 



lateral teeth approach to the form of the quadrate lobes in the 

 same position in the adult, the notches between them in the 

 earlier stage having been filled up by their increase in breadth. 

 In the adult the crenated margin becomes much smoother, the 

 crenations becoming so minute as to be all but obsolete. The 

 tubercles on the outer surface of the chelipeds are in the second 

 stage relatively smaller, but still distinct, and their arrangement 

 in longitudinal rows is more evident. In both stages there are 

 numerous small scattered tubercles on the surface of the carapace, 

 more prominent in the iirst stage than in the second ; in the 

 adult these are reduced to minute granulations. 



It is quite obvious that the second stage, represented in fig. 2, 

 could not possibly belong to Atelecydus, or indeed to any other 

 species than Cancer pagurus, and I think there is no doubt that 

 this form is derived from the first stage shown in fig. 1. But the 

 evident similarity of the form shown in fig. 1 to Atelecydus at 

 once suggests that the two genera are closely allied, and I was 

 led by this resemblance to compare the two British species more 

 carefully. As a result of this study 1 have come to the conclusion 

 that Atelecydus properly belongs to the family Cancridse, and 

 should be placed in close proximity to the genus Cancer, not in 

 the place hitherto assigned to it, in the family Corystidse. 



The points of resemblance between Atelecydus heteroclon and 

 Cancel- pagurus are namevows and obvious. In both the anterior 

 margin of the carapace is quinquedentate, one of the teeth being 

 median, and the two external forming the inner boundaries of the 

 orbits. The antero-lateral teeth require a detailed examination. I 

 have had for this purpose three specimens of Atelecydus, two males 

 3'7 cm. in diameter, one female 2*4 cm. I find there are really 10 of 

 these teeth in all, as in Cancer pagurus. The last or 10th, counting 

 that which forms the outer boundary of the orbit as the first, is 

 at the anterior extremity of the granulated ridge which borders 

 the dorsal surface of the carapace posteriorly. This tooth is 

 sometimes defined behind by a distinct indentation as well as in 

 front. This is the case on both sides of one of the males, on the 

 right side only in the other ; in the female the indentation is not 

 very distinct on either side. In Cancer j)agurus the tenth lateral 

 lobe or tooth, although much less marked than thereat and situated 

 on the postero-lateral margin, is defined posteriorlj^ by a distinct 

 indentation. 



In Atelecydus heteroclon the 1st, 3rd, 5th, 7th, and 9th teeth are 

 larger, the rest smaller. In my two male specimens the 2nd and 

 4th teeth are very small, almost rudimentary. Montagu, who 

 first defined the species, called it septemdentatus, and Stebbing 

 (' Crustacea,' Internal. Sci. Ser. 1893) states that there are 9 teeth 

 on the antero-lateral margin, and suggests that Montagu did not 

 include the point at each extremity of the series. It seems to me 

 more probable that Montagu omitted the 2nd and 4th on account 

 of their slight development, and also the 10th tooth, which might 

 be regarded as the extremity of the posterior granulated ridge. 



