IBLA AND SCALPELLUM. 283 



of the carinal latera ; the second group is characterised by 

 having a sub-carina and a large rostrum, and may be sub- 

 divided into two little groups ; viz., S. rostratum having 

 four pairs of latera, and S. Peronii and villosum having 

 only three pairs of latera : now the males, if classed by 

 themselves, would inevitably be divided in exactly the 

 same manner, namely, into two main groups, — the one in- 

 cluding the closely similar, sack-formed males of S. vulgare, 

 ornatum, and rutilum, the other the pedunculated males of 

 S. rostratum, Peronii, and villosum; but this latter group 

 would have to be subdivided into two little sub-groups, 

 the one containing the three-valved male of S. rostratum, 

 and the other the six-valved males of S. Peronii and S. vil- 

 losum. It should not, however, be overlooked, that the 

 two main groups of parasites differ from each other, far 

 more than do the two corresponding groups of species to 

 which they are attached ; and, on the other hand, that 

 the parasitic males of S. Pero?iii and S. villosum resemble 

 each other more closely, than do the two hermaphrodite 

 forms ; — but it is very difficult to weigh the value of the 

 differences in the different parts of species. 



Besides these general, there are some closer relations 

 between the parasites and the animals to which they are 

 attached; thus the most conspicuous internal character by 

 which Ibla quadrivalvis is distinguished from /. Cumingii, 

 is the length of the caudal appendages and the greater 

 size of the parts of the mouth ; in the parasites, we have 

 exactly corresponding differences. Out of the six species 

 of Scalpellum in their ordinary state, S. ornatum is alone 

 quite destitute of spines on the membrane connecting the 

 valves ; and had it not been for this circumstance, I should 

 even have used the presence of spines as a generic 

 character ; on the other hand, S. villosum, in accordance 

 with its specific name, has larger and more conspicuous 

 spines than any other species. In the parasites we have 

 an exactly parallel case ; the parasite of S. ornatum being 

 the only one without spines, and the spines on the parasite 

 of S. villosum being much the largest ! This latter species 

 is highly singular in having no caudal appendages, and 



