108 CARBONIFEROUS LAMELLIBRANCHIATA. 



It is very questionable whether it would not be advantageous to form a new 

 family to receive on the one hand those shells, now classed under the Mytilidae, 

 which show marked aviculoid characteristics, such as the group marked off by 

 Stoliczka; and, on the other hand, those genera at present classed under the 

 Aviculidse, as Ambonychia, Hall, Mytilarca, Hall, Oosseletia, Barrois, Pinna, 

 Pinnigena, Aviculopinna, Ptychodesma, Posidoniella, de Koninck (type, P. vetusta), 

 and others. I have removed the latter genus to the Mytilidae, from the Aviculidse 

 (vide p. 81). 



In support of this contention it may be stated that while Dr. John Young, 

 of the Hunterian Museum, Glasgow, has demonstrated the prismatic structure of 

 the shell in many of the members of this group, and also in the Aviculidse, he has 

 not been able to find it in Mytilus, Modiola, or Lithodomus, the most typical 

 members of the Mytilidae. 



Three of these Palasozoic Lamellibranchs are very closely allied to Myalina, 

 viz. Mytilarca, Hall, the sub-genus Plethomytilus, and Gosseletia, Barrois. They 

 all possess a striated hinge-plate and a mytiloid shape ; but these three genera 

 possess, in addition, hinge-teeth. These genera are found in the Devonian 

 beds of North America, and have also been recognised in England by the Rev. 

 G. F. Whidborne. 



Hall gives the following diagnosis of his genus Mytilarca in ' Geolog. Survey 

 State of New York, Palasontology,' vol. v, pt. i, Lamellibranchiata, i, 1884, p. xiv : 

 " 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." 



" Mytilarca, sub-genus Plethomytilus, Hall. Equivalve, mytiloid, gibbous 

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

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

 Test with concentric striae." 



The genus Oosseletia, Barrois, is thus defined by the same author : — " Shell 

 subtriangular, truncate on the anterior side, subalate on the posterior. Liga- 

 mental area wide, longitudinally striate. Cardinal teeth strong, situated under 

 the beak. Lateral teeth elongate. Surface marked by concentric strias." 



These three forms all differ from Myalina in possessing hinge-teeth as well as 

 a striated hinge-plate, while in general external form they are all very similar. 



The genus Myalina may be subdivided into two groups ; one, containing 

 M. Redesdalensis and M. Flemingi, being strongly angulated and much expanded 

 anteriorly. The other group, of which M. Verneuilii may be taken as the type, to 

 which most of de Koninck's species belong, are much flatter and more expanded, 



