xii SYNOPSIS OF THE GENERA. 



Pernopecten* Winchell. Shell like Pecten. Hinge with a central cartilage pit 

 and a crenulated hinge-plate on each side below the hinge-margin. This and 

 the following genus appear to be more nearly related to the genus Pecten than 

 to Aviculopccten. 



Crenipecten, Hall. Like Aviculopectcn in form. Hinge furnished with a series 

 of small cartilage pits throughout its entire length. Examples, pi. ix. figs. 12, 15. 



Aviculqpecten, McCoy, is emended to include those forms which have the hinge- 

 line usually shorter than the transverse diameter, and both ears well-defined. 

 Test ornamented with rays. Example, pi. i, fig. 10. 



Lyriopecten, Hall. Differs from the preceding in the short hinge-line and 

 very small anterior ear. Test usually ornamented with strong rays. Example, 

 pi. viii, fig. 8. 



Pterinopecten, Hall. Hinge-line long. Ears not well-defined, being simple 

 expansions or extensions of the upper lateral margins to the hinge-line. Test 

 ornamented with rays. Examples, pi. ii, fig. 18; pi. v, fig. 6. 



Pterinea, Goldfuss. Shell inequivalve, inequilateral : posterior side alate ; 

 anterior end nasute or auriculate. Ligament internal ; ligamental area longi- 

 tudinally striated. Cardinal teeth two or more. Lateral teeth linear oblique. 

 Posterior muscular impression large, situated on the post-umbonal slope. 

 Anterior muscular impression small, situated within the rostral cavity. Test 

 ornamented with rays. Examples, pi. xv, figs. 1-21 ; pi. lxxxiii, fig. 12. 



S. g. Vertumnia, Hall. Differs from Pterinea in having the right valve convex, 

 and the left fiat or concave. Hinge area narrow. Example, pi. xxiv, fig. 12. 



Actinopteria, Hall. Characterized from Pterinea in the absence of a broad 

 striated ligamental area and strong cardinal and lateral teeth. Right valve 

 sub-convex. Surface with fine rays. Examples, pi. xvii, fig. 26 ; pi. xviii, fig. 

 13 ; pi. xix, fig. 3. 



Ptychoptcria, Hall. Differs from Actinopteria in the nasute anterior extrem- 

 ity, and large straight wing marked by a strong longitudinal fold. Hinge- 

 line narrow, linear : furnished with one or two linear oblique cardinal and lateral 

 teeth. Surface with tine rays. Example, pi. xxiii, fig. 17. 



^See note, page SI. 



