NO. 1177. NORTB AMERICAN! LEPTONACEA—lJALL. 875 



jiiforinatiou, and, where only shell characters are known, chietiy on 

 those of the hinge. It is difficult at present to say what should be 

 done with Montacuta. According to the literature it has Lucinoid gills 

 and Thyasiroid hepatic digitations, while the shell is obviously Lepto- 

 noid. The anatomical combinations that the other groups would exhibit 

 are at i)resent unknown in many cases. It may be for the present most 

 convenient to place the Montacutas and Aligenas at the end of the list 

 with an unassigned value, as they certainly seem to lead uj) to the 

 Thyasiridw in spite of the differences of the gills. 



It does not seem practicable to as.sociate Sportella, Aiiisodonta, and 

 other genera, in which the soft parts are permanently retained within 

 the shell, with forms like Galeomma, in which they are exserted, cover- 

 ing a large part of the valves. The only data we have on Anisodonta 

 {quadrata) would indicate that the mantle edges are largely united, the 

 gills as in Thyasira {Cryptodon), but united behind the foot, and, con- 

 trary to the rule in the Leptonacea, the incurrent orifice, though not 

 developed into a siphon, is comjjlete and posterior. Yet the shell char- 

 acters merge so gradually into those of typical Anisodonta, and these 

 into those of Sportella, that one feels that they can hardly be widely 

 separated without more definite information. The interchanges of char- 

 acters and the multiplicity of forms separated by apparently trifling 

 details of structure make this group one of the most perplexing I have 

 ever tried to review. 



The following scheme is provisionally adopted. 



The name of the typical species follows the date of the genus. The 

 series begins with the most specialized forms. 



Family CHLAMYDOCONCHIDJE. 



Chlamydoeoncha Dall, 1884. C. orcutti Dall. 



Family GALEOMMATID^. 



Ephippodonta Tate, 1889. E. macdougaUi Tate. 

 Galeomma Turton, 1825. G. turtoni Sowerby. 



Sections: Amphilepida'DiiW, 12>^. G. |;o/i<a Deshayes. 

 Paralepida Dall, 1899. G. formosa Deshayes. 

 Lihratula Pease, 1865. L. plana Pease. 

 Solecardia Conrad, 1849. S. eburnea Conrad. 



Subgenera: Scintilla Deshayes, 1855. S. philippinenaia Deshayes. 

 Spaniorinus Dall, 1899. S. coasmanni Dall. 

 Scintillorbis Dall, 1899. S. erispata Fischer. 

 Vasconiella Dall, 1899. Vasconia Jeffrey aiana Fischer. 



Family SPORTELLID^. 



Sportella Deshayes, 1858. Paammobia dubia Deshayes. 



f Section : Fabella Conrad, 1863. F. conatricta Conrad. 



