284 REPORT ON ALBATROSS MOLLUSOA — DALL. 



above the septum. All were turgid; some had already burst aud 

 partly discharged their contents into the chamber ; others seemed on 

 the point of doing- so; the alcohol had coagulated the escaping ova in 

 situ, in the most perfect manner, the whole process thus being dis- 

 played. It is probable, as suggested by me in 188(1, that the chamber 

 serves to some extent as a marsupium or shelter for the ova and young, 

 and that they are not discharged into the surrounding element at once. 

 This is undoubtedly the case in Modiolarca. 



Family POROMYID.F Dall. 



Poromyidce Dall, Bull. Mus. Coinp. Zool., xn, p. 230, 1836; xvm, p. 452, June, 1889. 



The characters of this family are peculiar and exhibit au extreme 

 specialization in the matter of the si phonal septum and the develop- 

 ment of new breathing organs upon it, while the normal ctenidia have 

 become obsolete. As the matter has been discussed with, as I believe, 

 erroneous conclusions and assumptions by Dr. Paul Pelseueer in his ac- 

 count of the Anatomy of Mollusks in the series of reports ou the scien- 

 tific results of the Challenger expedition, it seems well to include here 

 the data aud conclusions to which a study of the Fish Commission and 

 other material has conducted the writer. This has already been stated 

 elsewhere, in the appendix to my Report on the Blake Mollusca, Part II, 

 but as that publication is likely to be somewhat restricted in its au- 

 dience, compared with the IT. S. Fish Commissioner's reports, it is hardly 

 necessary to apologize for introducing the same matter here ; especially 

 as it is based directly on the Albatross collection. Part of the data will 

 be found under the heads of the several species, the rest is here assem- 

 bled for reference. 



In 1886 I separated from Poromya the forms which, when adult, have 

 the hinge teeth obsolete, under the name of Cetoconcha. This group in- 

 cluded not merely those with a double posterior row of modified septal 

 orifices on each side, such as C. bulla, the type, and C. margarita, but 

 also certain species of Poromya, in which the hinge teeth are feeble or 

 obsolete in the adult, while in the typical Poromya they continue strong. 

 I called attention to the fact that the soft parts of these species did not 

 differ essentially from Poromya, but hardly felt justified in separating 

 them from the typical Cetoconcha'. It is probable that it would be better 

 for them to form a section of Poromya which may be called Cetomya; 

 while the typical Cetoconcha may perhaps be generically separated from 

 Poromya. The group in question was named Silenia by Mr. E. A. 

 Smith, in Tiis report on the Challenyer Lamellibranchs, but that name 

 had already been used in zoological nomenclature, and so was preoccu- 

 pied. The observations of Pelseneer ou the anatomy of Silenia leave 

 no room for doubt that it is identical with Cetoconcha, as represented 

 by its type and by C. margarita. Now that wider research has shown 

 more clearly the characters of Poromya and Cetoconcha, the attempt of 

 1880 to diaguose both forms in a single definition seems confused, but 



