MUSEUM OF COMPARATIVE ZOOLOGY. 447 



typical Cetoconcha may perhaps be generically separated from Poromya. The 

 group in question was named SUenia by Mr. E. A. Smith, in his Report on 

 the Challenger Lamellibranchs, but that name had already been used in zoo- 

 logical nomenclature, and so was preoccupied. The observations of Pelseneer 

 on the anatomy of SUenia 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 1886 to diagnose both forms in a single definition seems con- 

 fused, but with this explanation it should be clear enough that the facts were 

 observed and recorded in members of each group, and that the apparent con. 

 fusion in the diagnosis resulted from a feeling of conservatism in the matter 

 of subdividing genera; a proceeding which has, of late years, on some occasions 

 been so shamefully abused. 



The researches of the U. S. Fish Commission have added some most inter- 

 esting and peculiar species of this family, which will be more fully described 

 and figured in the Report on the Voyage of the Albatross, now in preparation. 



Cetoconcha bulla Dall (I. p. 283). The description of 1886 merely requires 

 the addition of the statement that the lamella? described are subtubular, and 

 form the lips to the septal orifices. In using the term " ventral surface," for 

 the under side of the septum and " body cavity," the reader will not be misled 

 into the supposition that the visceral mass was the " body " intended ; for, 

 though the words may have been ill chosen, the relations of the visceral mass 

 were clearly stated, although the very important relation of the upper chamber 

 to the anal siphon was not understood at the time. 



There is in this species a distinct bunching of the muscular fibres at the 

 posterior outer corners of the septum, from which points they extend in a 

 somewhat radiating manner. The soft parts, though more rotund, and with 

 a different number of septal orifices, resemble sufficiently those of Cetoconcha 

 Sarsii Smith, as diagrammatized by Pelseneer. For each orifice two lamellse 

 are usually counted in the Report of 1886, as the lips of the septal orifices 

 generally appear paired and arched, forming a segment of a circle. In C. bulla, 

 in the anterior series there are five orifices on each side; the inner posterior 

 series have three to five, and the outer posterior series two, or possibly three 

 orifices each. The number of posterior orifices is not the same in the two 

 specimens of C. bulla examined. 



In none of the specimens of Cetoconcha examined by me were the inner 

 ends of the four posterior series so widely separated as in Pelseneer's figure 9, 

 of SUenia Sarsii. They always seemed closer together, and more evidently 

 radiating from a central elevation on the septum behind the foot. But too 

 much stress must not be laid on the discrepancies of these diagrams, which 

 are not, and do not appear to be intended for. exact and complete portraits. 

 In this species a trace of the lateral arrangement of the siphonal muscles re- 

 mains, while, compared with Cuspidaria, the septal muscles are still in a 

 transitional state. 



Cetomya elonrjata Dall (I. p. 283). In the single specimen of this form, the 



