32 COLPODASPIS. 



the fact that the foot is attached to the body by a somewhat narrow 

 stalk a feature which it shares with most Prosobranchs. Gwyn 

 Jeffreys even informed him that he was inclined to consider 

 Colpodaspis as the young of Cyprcea europcea a view which now, 

 at any rate, can no longer be entertained. 



In spite of our ignorance of the anatomy of Colpodaspis we may, 

 however, as a result of the above observations, be certain that 

 Colpodaspis is a true Opisthobranch. It resembles various Cephal- 

 aspidea in the pleuropodial expansions of its foot (cf. Haminea), in 

 the posterior appendage of the mantle (Haminea, Philine), in its 

 inflated shell (Haminea, Utriculus), and in its radula (Philine). On 

 the other hand it resembles the-Notaspidea, and differs from the 

 above types of Cephalaspidea, in the great extent of the mantle and 

 in the form of the head and tentacles. In the latter point it again 

 resembles the Anaspidea, for in the young Aplysia, as I have often 

 observed, there is only one pair of tentacles (the anterior one) for a 

 considerable period, and these are grooved just as in Celpodaspis 

 and Pleurobranchus. These various points of resemblance are all 

 explicable if we regard Colpodaspis as a very primitive type of 

 Tectibranchiate mollusk, belonging indeed to the Cephalaspidea, but 

 retaining in an unspecialized condition an unusual number of those 

 primitive characters which the common ancestors of the Cephalas- 

 pidea and Notaspidea alike possessed. It supplies an ind ubitable con- 

 necting-link between these two great subdivisions of the Tecti- 

 branchia; but it belongs to the group Cephalaspidea, in spite of the 

 inappropriateness of the name, owing to its acquisition of pleuropodial 

 expansions and a posterior pallial appendage two associated 

 features which are especially characteristic of this group. 



The question still remains open whether or not the creature de- 

 scribed by Sars and myself has assumed its adult features. Fischer 

 has suggested that Colobocephalus costellatus and Colpodaspis pusilla 

 are possibly only young stages of Philine or of neighboring genera 

 of Tectibranchs, owing to the radula in these two types resembling 

 very closely the radula of certain species of Philina (velutinoides, 

 lima, angulata). This theory, however, is in my opinion, altogether 

 untenable in the case of Colobocephalus, which, beyond the radula, 

 presents no particularly cephalaspidean, or even Opisthobranchiate, 

 features. The probability, on the other hand, that the Philinidse 

 have been derived phylogenetically from a Colpodaspis-like ancestor 

 is sufficiently great to render Fischer's view in this case worthy of 



