1908.J ASPIDOBRANCH GASTROPOD MOLLUSCS. 879 



As to the larger questions of the relationship of the Xeritidae 

 to other groups of the Rhipidoglossa, and whether they can be 

 regai'ded as repi-esentatives of the stock from which the Pectini- 

 branchia were derived, I think that some evidence can be offered 

 from the foregoing pages. The Neritidse ai-e commonly held to 

 be a highly specialized and at the same time an annectant group 

 leading to the Architaenioglossa. Such a view, though it may 

 contain an element of truth, cannot, in my opinion, be held 

 without considerable modification. 



To deal first with the position of the Neritida? among the 

 Rhipidoglossa. Thiele admits that he cannot assign them a 

 satisfactory position. He is inclined to derive them from the 

 Trochidje, but also detects resemblances to the Acmseidse ; as he 

 rightly observes, they cannot be derived from the latter family, 

 for it is docoglossate. 



If we compare the Neritidse with the rest of the Rhipidoglossa 

 we see that they possess a number of primitive characters common 

 to the whole group. Such are, the short snout, without pre- 

 tentacular or post-tentacvilar elongation ; in the nervous system, 

 the presence of a labial commissure, of elongated buccal ganglia, 

 of a long cerebral commissure, of elongate pedal nerve-cords 

 sheathed in ganglion -cells. The pharyngeal bulb is large, situated 

 far forward in the head and embraced by the cerebral com- 

 missure. The ventricle of the heart is traversed by the rectum, 

 and although that of the right side is much reduced there are two 

 auricles. The ctenidium is typically bipectinate and is less 

 modified than in the Trochidse, for the lower gill-lamellse are 

 equal in size to the upper. 



The characters indicating a higher degree of specialization than 

 in other Rhipidoglossa are as follows : — The eyes are closed, 

 the supra-intestinal nerve is reduced or even absent, and the 

 left symmetrical pallial nerve takes a principal share in the 

 innervation of the ctenidium. The subintestinal ganglion is 

 closely approximated to the right pleui-al and is vmited by a very 

 short zygoneurous branch with the latter. A direct commissviral 

 connection is established between the right and left pleural 

 ganglia. Only a single functional kidney — the left post-torsional 

 — is present, its fellow of the right side having been converted 

 into the gonaduct. The accessory genital ducts are extremely 

 complicated and in the female are diaulic or triaulic. The 

 visceral spire is reduced and the animal has acquired a secondary 

 symmetry emphasized by the presence of a left as well as a right 

 columellar muscle. In possessing a single (left) ctenidium the 

 Neritidte stand on the same level of oi-ganization as the Trochidte 

 or Turbonid^e. 



It might be concluded from all this that the ISTeritidse are 

 nothing more than Rhipidoglossa which have been specialized in 

 certain directions while retaining many of the primitive features 

 characteristic of all the Aspidobi-anchia, and that they so far 

 resemble the monobranchiate forms (Trochidae and Turbonidse) 

 Proc. Zool. Soc— 1908, No. LYI. 56 



