30 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



section of the foregoing genus, to which it is closely related. It differs, 

 ^however, in its more nearly equi valve character, much less oblique, truncato- 

 suborbicular form, broader cardinal margin, and edentulous hinge. It also 

 differs from all of the sections of that group, excepting Electroma and Pseud- 

 optera, in having its posterior wing very short, or obsolete, while it is very 

 unlike those types in nearly all other characters. 



The name Pseudoptera was proposed by me for a peculiar Cretaceous 

 group, the hinge and interior of which are little known. The species of this 

 type have generally been described under the name Avicula; but they differ 

 very materially from the typical forms of that genus in having no well-defined 

 wings or byssal sinus. I proposed, and now retain, this group, provisionally, 

 as a section of Pterin (— Avicula); but it may be even generically distinct. 

 In first indicating this section, in 1873, I mentioned as its type Avicula 

 anomala, Sowerby ; that being the best known species presenting the general 

 form of the group. It is barely possible, however (though very improbable), 

 that the hinge of that shell, which is entirely unknown, may present the 

 characters of Melina or Gervillia, and thus make my name Pseudoptera, 1873, 

 a synonym of an older genus. If so, I would beg leave to retain it for 

 the group including Avicula fibrosa, M. & H., which I really had more in 

 view at the time than Sowerby's species. The use, de novo, of a name- 

 originally proposed for a type that proves fo belong to a previously-estab- 

 lished genus, though generally undesirable, is still in accordance with the 

 usages of many high authorities in natural history. 



The group Oxytoma was also originally proposed by me as a subgenus 

 under Pteria (— Avicula); though I distinctly stated that it forms a transi- 

 tion from that genus to Eumicrotis, a section of Pseudomonotis of Beyrich. 

 The opinion has been expressed by Dr. Stoliczka that it ought rather to 

 stand as a section of Pseudomonotis than of Pteria. It is so evenly balanced 

 in its affinities between Pteria and Pseudomonotis that it might, with almost 

 equal propriety, be ranged as a section of either. In obliquity and general 

 form, as well as in the development of its wings, it agrees best with the 

 former; while, in the nature of its byssal sinus, it agrees exactly with Pseu- 

 domonotis. I leave it as a section of Pteria; but, at the same time, place it 

 farthest from the typical group of that genus 



It may be thought that the existence of such an intermediate type 

 warrants the union of the whole (including Pseudomonotis) under the one 



