MUSEUM OF COMPARATIVE ZOOLOGY, 451 



General Summanj. — Premisiufr that, in Lliia arlicio Uio, word clmidium is 

 einployccl to designate tlie normal typical gill of Pelecyi)od.s, in any of its 

 modifications, as opposed to tenii)orary or local branchial organs of different 

 origin, the facts just reviewed may be briefly summarized. 



1. In many groups of IVlecypods the ctenidia are more or less united be- 

 hind the foot, 80 as to divide the peripedal chamber into an upper, or anal, and 

 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 partition. 



2. In other forms, the siphonal septum is extended forward to form a 

 partition, either (A) unmodified (Cardium); (B) thickened without orifices 

 (Verticordia); (C) assuming a retractile function (Cuspidaria) with orifices; 

 (D) only partially retractile (Dermatomya) with single lateral series of orifices; 

 or (E) with an incomplete double lateral series of orifices {Cetoconcha). In 

 these cases the breathing organs may be (A) unmodified ctenidia, (B) depau- 

 perated adnate ctenidia, (C) the general surface of the septum without ctenidia 

 or specialized lamella), (D) with only specialized flat lamellsc, or (E) with spe- 

 cialized subtubular proliferations. In these cases the structure of the septum 

 appears to be wholly independent of the ctenidia, though in Verticordia they 

 are adnate upon its surface, 



3. There is one form (^Lyonsiella ahyssicola) 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. radiatd) the septum is of the kind described in paragraph 1. 



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

 in youth, and open with the attainment of sexual maturity. 



5. The anal chamber, as indicated in 1886, seems to fill the office of a 

 marsupium. 



6. The tissues of the septum may therefore be derived from structures di- 

 verse 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 assumption, 

 that the septum is essentially ctenidial in its origin, is unwarranted, and his 

 group SeptibrancJiia, 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 

 and their variations in forms otherwise closely related, the name has no claims 

 for adoption either in a strictly genealogical or an eclectic system of classifica- 

 tion. It miiy be added, that the "proof" that Poromya and 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 pub- 

 lished has pretended in any way essentially to modify. This classification, 



