xiv SYNOPSIS OF THE GENERA 



of the shell. Wing and binge extended posteriorly. Test marked by concen- 

 tric striae. Example, pi. xxii, fig. 20. 



Palaopinna, Hall. Shell similar in outline to Pinna, gaping in front. Tesi 

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

 than in the ordinary Pinna. Hinge-line simple. Example, pi. xxv. fig. I*. 



Ectenodesma, Hull. Resembles Grlyptodesma in outline, except that the ante- 

 rior wing is more produced anil both wings more acute at their extremities. 

 'Test ornamented with rays. Example, pi. xxiii. fig. 30. 



Byssopieria, Hall. Shell erect, equivalve. Alate posteriorly, truncate, and 

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

 21. 22; pi. lxxx fig. I I. 



Mytilarca, s. g. Plethomytilw, 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 stria'. Examples, pi. xxx. figs. 5, 7 : pi. xxxi,figs. 2. '■). 7. 



Mytilarca, 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. Examples, pi. xxxii, figs. 9, 17, 19, 20. 



Gosselet/ia, 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. tigs. 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 

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

 of the shell. Beaks terminal. Examples, pi. xxxiii, figs. 10, 17; pi. lxxxvii. 

 fig. 0. 



