876 PROF. G. C. BOURNE ON THE [Nov. 17, 



arranged in several concentric layers as in d, and in other cases, 

 as in c, a concretion is made up of an aggregate of several smaller 

 concretions. As the crystal sac is always full of these concretions 

 in Nerita and Paranerita they must be of some importance, 

 and I think that they are connected with the formation of an 

 external calcareous layer of the , egg-shell. The egg-cases of 

 Nerita and Paranerita are not known, but those of Septaria 

 boiogainvillei have an external calcareous envelope which is readily 

 dissolved in dilute acids leaving a horny layer beneath. The 

 crystal sac is very small in Septaria, and this may account for 

 the calcareous layer being very thin. If this surmise be correct, 

 the " crystal sac " is a calcigenous gland, as hinted in a footnote 

 by Thiele, though for some unexplained reason he prefers to call 

 it the uterus. 



Before bringing this part of my work on the Neritacea to a 

 close, I inay conveniently discuss the various questions arising 

 out of the facts enumerated. In the first place, there is the 

 question of the inter-relationship of the existing members of the 

 Neritidte. Leaving ScuteUina out of the question, because I 

 have not been able to obtain specimens of this genus, I have to 

 justify my subdivision of the members of the family into the 

 groups Nerita, Paranerita, Septaria, and Neritina. The fact 

 that the female Neritina {sensu restricto) is diaulic and Para- 

 nerita triaulic is in my opinion sufficient to separate these forms 

 from one another. Further than this Neritina resembles Nerita 

 more closely than Paranerita, not only in being diaulic, but also 

 in the characters of the epididymis, in having the epididymis 

 and spermatophore-sac restricted to the mantle-region instead of 

 projecting backwai'ds into the coelom, and in the position of 

 the receptaculum seminis on the vaginal canal. Neritina, again, 

 is more specialized than any other of the Neritid8e in that it has 

 lost the supra-intestinal nerve and the oviduco-ccelomic funnel. 

 The evidence of comparative anatomy the}'efore points to its 

 having been evolved independently of Paranerita from a marine 

 Nerita stock, and this conclusion is strengthened by a consider- 

 ation of the evidence afibrded by distribution in space and time. 

 Assuming, as we are amply justified in doing, that all estuarine 

 and freshwater forms are descended from m.arine Neritidpe, 

 the various species of Neritina inhabiting rivers debouching into 

 the Mediterranean, Caspian, and ISTorthern European seas must 

 have been derived from a marine form inhabiting those seas. At 

 the present time no member of the genus Nerita {s. stricto) is 

 found in any of them. Neritina viridis, it is true, is found in 

 the Mediterranean, but this is probably a fluviatile form which 

 has found its way back to the sea, for even N. fiuviatilis occurs 

 in brackish and sometimes in salt water. The ancestral marine 

 forms must therefore be looked for in geological strata, and it is 

 significant that, whereas Neritina is common in Tertiary deposits 

 and extends back as far as the Lias, the most recent fossils 



