516 DR WHEELTON HIND ON 



I question whether Beushausen's species should be referred to Cardiola. It is clearly 

 more nearly allied to Pararca, Hall. Hall shows that the posterior side of the shell 

 is the longer. 



Genus Mytilarca, Hall, 1870. 



Mytilarca, Hall, 1870, Prelim. Notice Lamell. Shells, p. 19. 



,, Hall, 1884, Oeol. Surv. Neio York, vol. v., Pal., i., Lamell., pt. ii., p. xiv. 



„ Miller, 1889, North American Geol. and Pal., p. 493. 



Generic Characters. — " 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." 



Mytilarca, sub-gen. Pie thorny tilus, Hall, 1884. — "Equivalve, mytiloid, gibbous. 

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

 transverse. Lateral teeth small and oblique. No cardinal teeth have been observed. 

 Test with concentric striae." 



Observations. — It is very questionable whether the sub-genus Plethomy tilus is 

 necessary. In Naiadites, Myalina, and other mytiliform genera, it is found that the 

 species have been formed by differences in the length of the hinge line. For example, 

 M. Verneuilli has an elongate hinge line, and M. Flemingi a very short one. 

 N. modiolaris has a long one, and JY. carinata and N. triangularis much shorter. 



Mytilarca is closely related to Ambonychia, but there seems to be no byssal notch, 

 judging from the figured specimens in Hall's work. 



Mytilarca is well represented in Devonian rocks. 



Mytilarca (Plethomy tilus) mytilimeris, Conrad, sp. 1842. (PI. II., figs. 11, 12.) 



Inoceramus mytilimeris, Conrad, 1842, Jour. Acad. Nat. Sc, vol. viii. p. 241. 



Mytilus mytilimeris, Salter, 1848, Mem. Geol. Surv. Gt. Britain, vol. ii., pt. i., p. 364, pi. xx. 



figs. 7, 8, 9. 

 Plethomytilus mytilimeris, Miller, 1889, North American Geol. and Pal., p. 503. 



Specific Characters. — Shell of moderate size, compressed, ovate. No anterior side. 

 The valves are deepest at the anterior border, which descends almost vertically down- 

 wards at first, then forms the well-rounded inferior margin. The posterior margin 

 is slightly oblique, almost straight, forming a continuous curve with the lower border, 

 and almost a right angle with the hinge line above. The hinge line is short and 

 straight. The umbones are pointed, twisted forward, and terminal, projecting over the 

 anterior border. The postero-superior angle is compressed and subalate. 



Interior. — I am able to give no details, except that the hinge plate was thick in the 

 neighbourhood of the umbo. 



IvMerior. — The surface seems to have been almost smooth. 



