280 BULLETIN OF THE 



Family POROMYID.E Dall. 

 Genus POROMYA Forbes. 



Poromya Forbes, 1844 ; Emhla Loven, 1846. Type P. anatinoides Fbs. {= P. granu- 



lata Nyst.) 



Shell gaping a little behind, granulose externally under a thin epidermis, in- 

 ternally with an internal cartilage in a stout posteriorly directed fossette over 

 which a linear external ligament extends from under the beaks backward over 

 the cartilage to the posterior end of the hinge-line ; before the cartilage in the 

 right valve is a stout cardinal tooth, generally notched in front; in the left 

 valve is a small sunken triangular tooth in front of the fossette, and a long 

 distant posterior lateral tooth lies behind the beak. There is a very slight 

 indentation of the pallial line, the foot is long and cylindrical, the siphons 

 rather short, surrounded with a I'ringe of rather stout tentacles. There is no 

 ossicle. Gills as in Cetoconcha, with no free branchiae. The interior of the 

 shell is faintly pearly under a wash of non-perlaceous substance. 



Section CETOCONCHA Dall. 



Shell differing from Poromya proper by the cartilage being almost external 

 and the fossettes diminished in size and upturned, the external ligament con- 

 sequently nearly obsolete ; the dentition obsolete except the cardinal tooth of 

 the right valve, which itself is sometimes absent in the adult, though observ- 

 able in the young shells ; other shell characters miich as in Poromya, The foot 

 is compressed and hatchet-shaped, grooved behind ; the mouth has two large 

 superior palpi and two (or none) small inferior palpi not modified as gills. 

 The foot stands in a socket as in Verticordia and Cuspidaria. On the ventral 

 surface of the body, behind the foot, are two (sometimes four) rows of less than 

 semicircular lamellae closely adjacent to each other and firmly fixed to the sur- 

 face by the whole base of each lamina. There is one row on each side ■with a 

 shorter supplementary outer row in other cases. They radiate forward in a 

 curve from a point a little distance behind the foot, and may quite or not quite 

 meet at this point. In C elongata I found no inferior palpi, a state of things 

 perhaps due to injury, though the specimen seemed perfectly preserved; the 

 other species had them. In all there was a row of similar lamellae to those 

 above described, starting on each side from behind or under the inferior palpus 

 of that side, or the place in front of which it should have been, and extending 

 backward in such a curve as would, if prolonged, have joined its posterior end 

 to the anterior end of the row coming from behind the foot. The lamellae are 

 not connected by a raphe. These lamellae represent the branchiae of ordinary 

 Pelecypods, and if even these are absent, as seems possible, in Cuspidaria, it 

 is difficult to doubt that we have a progressive series: in Cuspidaria none ; in 



