xiv SYNOPSIS OF THE GENERA. 



ol tlir .<li<-ll. WiuLi anil liinge extended posteriorly. Test marked by concen- 

 tric stria'. Example, pi. xxii, fig. 26. 



Poheophina. Hall. Shell .similar in outline to Pinna, gaping in trout. Test 

 marked li\ fine radiating lines. Body more convex and test with finer rays 

 tiian in tlie ordinary Pinna. Hinge-line simple. Example, pi. xxv, fig. 18. 



KctenoJesma, Hall. Kesendjles Glyptodesma in outline, except that the ante- 

 i-i<jr wing" is uiore produced and both wings more acute at their exti'eraities. 

 Test ornamented with rays. p]xample, pi. xxiii. fig. 30. 



Bijssoptiria, Hull. Shell erect, equivalve. Alate posteriorly, truncate, and 

 with a nasute projection in front. Surface radiated. Examples, pi. xxiii, figs. 

 21, 22: pi. Ixxx fig. 11. 



Mijtilarca, s. g. Pleihomijlilus, Hall. Equivalve, mytiloid, gibbous. Shells 

 with a finely striated ligamental area. Posterior side subalate. Hinge-line 

 transverse. Lateral teeth small, oblique ; no cardinal teeth have been observed. 

 Test with concentric stride. Examples, pi. xxx, figs. 5, 7 ; pi. xxxi, figs. 2, 3, 7. 



Mijtilarca, Hall. Shell equivalve, inequilateral and mytiliform with terminal 

 beaks and short hinge-line, which is bordered by a flattened, longitudinally 

 striated, ligamental area of greater or less extent. Cardinal teeth small, 

 situated beneath the beak. Posterior teeth small and oblique, situated at the 

 post-cardinal extremity of the hinge. Test free from radii, with a single 

 known exception. Example.s, pi. xxxii, figs. 9, 17, 19, 20. 



Gosseleltia, Barrois. Shell subtriangular, truncate on the anterior side, sub- 

 alate on the posterior side. Ligamental area wide, longitudinally striate. 

 Cardinal teeth strong, situated under the beak. Lateral teeth elongate. Sur- 

 face marked by concentric striae. Examples, pi. xxxi, figs. 10-16. 



Modiola, s. g. Mytilops, Hall. The fossils of this genus resemble Modiola and 

 Lithodomus in external form, and may also be compared with the fossil genus 

 Myoconcha. Hinge-line narrow, oblique, extending about one-half the length 

 of the shell. Pxaks ti'rminal. Examples, pi. xxxiii, figs. 16, 17; pi. Ixxxvii, 

 fig. 9. 



