286 REPORT ON ALBATROSS MOLLUSCA DALL. 



the membranes of the septum, etc., are extremely delicate. The use of 

 too concentrating hardening agents or the incautious touch of a probe 

 *will produce lesions which may be indistinguishable from normal fis- 

 sures. To make sure that nothing of this sort shall happen, it is neces- 

 sary to float the soft parts in a cup of water and turn them about with 

 delicate forceps. This is not convenient in all respects for observation, 

 but with time and patience the characters may be made out. 



Young specimens of this species show the lamellar areas as usual, with 

 the depressions above them, in the floor of the upper chamber, but the 

 fissures are not open; a fact which leads me to believe that they appear 

 only with maturity. A very delicate membrane seems to hold the dis- 

 tal margins of the lamellae together, so that a delicate probe passes over 

 without separating them. 



General considerations on the nature of the septum in Poromyidw and 

 Guspidariidce. — The facts above and others elsewhere stated indicate 

 that the septum in these groups is essentially a prolongation forward 

 and a specialization of the ordinary siphonal septum. The septum, as 

 pointed out in Gardium, may be so prolonged, while the normal gills 

 are fully developed and unconnected with it. In Verticordia it may be- 

 so prolonged, and may have acquired a conspicuously fleshy texture 

 without fissures, while the gills lie prone upon it, more or less aduate. 

 The muscular apparatus by which the siphons are retracted, and whose 

 normal points of origin are at the side of the ordinary septum, appear 

 to be shifted to its surface. Different species show this process in dif- 

 ferent stages of completion; and in the only case among the Poromyas, 

 where the fibers follow the normal direction in other Pelecypods, the 

 septum is destitute of the muscular structure which is so prominent in 

 the other Poromyas. In the specialization of the septum the muscula- 

 tion develops from behind. When branchial lamina? are situated upon 

 the septum, and are not simply the ordinary cteuidia in an adnate con- 

 dition, the addition of a second series is made at the posterior end, and 

 all the branchial areas appear to receive their blood supply from be- 

 hind. 



There is not a particle of evidence to prove that the septal branchial 

 lamella? of Poromya and Getoconcha are homologous with the cteuidia 

 of Verticordia, Lyonsiella, Pema or Gardium. The fact that Guspidaria 

 has neither cteuidia nor any specialized laminae on the septum lends 

 probability to the assumption that the two series represent a parallel 

 among these Pelecypods to the cteuidia and the peripedal laminae in 

 Aemaa, Scurria, and Patella, among the Docoglossa. That is, that the 

 septal laminae are a new and special development, which functionally 

 replace, but are not homologous with, the original ctenidia. If this 

 view is doubted, the burden of proof lies upon those who call it in 

 question. 



It may be asked whether any hypothesis can be suggested by which 

 this peculiar specialization may be accounted for. The law of economy 



