XIV 
SYNOPSIS OF THE GENERA. 
of the shell. Wing and hinge extended posteriorly. Test marked by concen¬ 
tric striae. Example, pi. xxii, fig. 26. 
Palaopinna, Hall. Shell similar in outline to Pinna, gaping in front. Test 
marked by fine radiating lines. Body more convex and test with finer rays 
than in the ordinary Pinna. Hinge-line simple. Example, pi. xxv, fig. 18. 
Ectenodesma, Hall. Resembles Glyptodesma in outline, except that the ante¬ 
rior wing is more produced and both wings more acute at their extremities. 
Test ornamented with rays. Example, pi. xxiii, fig. 30. 
Byssopteria, Hall. Shell erect, equivalve. Alate posteriorly, truncate, and 
with a nasute projection in front. Surface radiated. Examples, pi. xxiii, figs. 
21, 22 ; pi. lxxx fig. 11. 
Mytilarca, s. g. Plethomytilus, 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 striae. Examples, pi. xxx, figs. 5, 7 ; pi. xxxi, figs. 2, 3, 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. 
Gosselettia, 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. Beaks terminal. Examples, pi. xxxiii, figs. 16, 17; pi. lxxxvii, 
