288 REPORT ON ALBATROSS MOLLUSCA DALL. 



(1) In many groups of Pelecypods the ctenidia are more or less 

 united behind the foot, so as to divide the peripedal chamber into an 

 upper or anal aud a lower or branchial portion. In these cases (Perna, 

 Modiolarca, for example) there is no important modification of the 

 structure of the gills, and the septum is truly branchial in character, 

 and the siphonal septum takes no part in the formation of the par- 

 tition. 



(2) In other forms, the siphonal septum is extended forward to form 

 a partition either (A) unmodified (Gardium), (B) thickened without ori- 

 fices (Verticordia), (0) assuming a retractile function (Cuspidaria) with 

 orifices, (D) only partially retractile (I)ermatomya) with single lateral 

 series of orifices, or (E) with an incomplete double lateral series of 

 orifices (Cetoconclia). In these cases the breathing organs may be (A) 

 unmodified ctenidia, (B) depauperated adnate ctenidia, (C) the general 

 surface of the septum without ctenidia or specialized lamella, (D) with 

 only specialized flat lamellae, or (E) with specialized subtubular pro- 

 liferations. In these cases the structure of the septum appears to be 

 wholly independent of the ctenidia, though in Verticordia they are ad- 

 nate upon its surface. 



(3) There is one form (Lyonsiella abyssicola) in which the siphonal 

 septum and the ctenidia are stated to be mutually attached, so that the 

 septum may be said to be of a compound formation, though in another 

 species of the same genus (L. radiata) the septum is of the kind de- 

 scribed in paragraph 1. The first statement stands in need of confirma- 

 tion. 



(4) The orifices in the septum of Poromya seem to be closed, or partly 

 closed, in youth, aud open with the attainment of sexual maturity. 



(5) The anal chamber, as indicated in 18SG, seems to fill the office of 

 a marsupium. 



(6) The tissues of the septum may therefore be derived from struct- 

 ures diverse in their origin, in some cases ctenidial and anterior, in 

 others siphonal and posterior. 



(7) Finally, from these facts it is evident that Dr. Pelseneer's as- 

 sumption, that the septum is essentially ctenidial in its origin, is un- 

 warranted, and his group Septibranchia, as defined by him, is founded 

 on an error of observation. While as a group-name it may be used to 

 indicate features of structure whose origin he misunderstood, yet, from 

 the purely adaptive nature of these features aud their variations in 

 forms otherwise closely related, the name has no claims for adoption 

 either in a strictly genealogical or an eclectic system of classification. 

 it may be added, that the "proof" that Poromya aud Silenia { — Ceto- 

 concha) are more nearly related to each other than to Cuspidaria, which 

 Dr. Pelseneer claims to be his work (op. cit., p. 25), had been published 

 by me more than two years previous to the appearance of his paper, 

 and exemplified in the classification I then proposed; a classification 

 which nothing since published has pretended in any way essentially to 



