INVERTEBRATE PALEONTOLOGY. 29 



long posterior lateral tooth in one or both valves; muscular scars two or 

 more, one very large, the others small; pallial line simple; byssal sinus gen- 

 erally more or less defined under the anterior car of the right valve; surface 

 marked with radiating costaa or striae, concentric imbricating laminae, or 

 nearly smooth, the markings being generally stronger on the left valve. 



The foregoing synonymy and diagnosis are intended to include four 

 sections, distinguished mainly by general form, the greater or less develop- 

 ment of the wings, the more or less strongly-defined character of the byssal 

 sinus, &c, as follows: 



1. pteria, Scopoli (typical). 



Shell generally very oblique, with wings well developed ; the 

 posterior one long, and defined by a deep marginal sinuosity ; byssal 

 sinus under, the anterior wing of the right valve moderate. — (Type 

 as already stated.) 



2. electroma, Sloliczka. 



Shell differing from the typical section in being more inequivalve 

 (the right valve being generally more flattened), and in having a 

 shorter hinge, without well-developed alations ; the posterior wing 

 being very short or obsolete, and not defined by a marginal sinus ; and 

 the anterior very small, or not projecting beyond the anterior mar- 

 gin, but defined in the right valve by a deep, sharply-cut sinus, close 

 up under the end of the hinge. — Avicula Smaragdina, Reeve. 



3. pseudoptera, Meek. 



Shell more or less obliquely subtrigonal or subovate; hinge 

 short, compressed ; anterior wing short, not defined ; posterior abbre- 

 viated, compressed, and nearly or quite without any marginal sinuosity 

 below it; anterior margins sometimes a little sinuous near the middle, 

 but without any byssal sinus under the anterior wing. — -Avicula 

 anomala, Sowerby. Pinna Jibrosa, M. & H. 



4. oxytoma, Meek. 



Shell with nearly the general outline of the typical form, but 



usually less oblique, and more inequivalve, with the byssal sinus very 



deeply and sharply cut, close up under the anterior auricle of the 



right valve. — Avicula Mvnsteri, Bronn. 



The group for which Miihlfeldt, in 1811, proposed the name Margariti- 



pkora (— Meleagrina, Lam., 1812), is, by several authors, also included as a 



