488 DR WHEELTON HIND ON 



Localities. — Middle Bala, Shalloch Mill ; Upper Bala, Star-fish bed. 



Observations. — A much narrower, less transverse variety, with a more elongate 

 narrow umbo, than B. radiata, and the radiating ribs are fewer and larger. Unfor- 

 tunately, the material is not in a good state of preservation, and the specimen is 

 anything but perfect. A single example of what I take to be the same species occurs 

 in the Star-fish bed. 



Genus Ambonychia, Hall, 1847. 



Ambonychia, Hall, 1847, Geol. Survey New York, Pal., vol. i. p. 163. 



,, Miller, 1889, North American Geology and Paleontology \ p. 460. 



Generic Characters. — Shell equivalve, inequilateral, compressed and subalate 

 posteriorly. Anterior side inflated and adpressed. Umbones raised, incurved, projecting 

 forwards. Cardinal line straight. Sinuate anteriorly for the byssus. 



Interior. — The hinge has a cardinal tooth below the umbo, and posteriorly two 

 or three narrow, elongate, slightly diverging teeth. 



Exterior. — The surface is almost smooth, or with concentric lines. 



Observations.- — I have followed Ulrich in placing the well-known form A. radiata 

 of authors in his new genus Byssonychia. He states that an examination of the 

 type Ambonychia bellistriata. shows that it has no byssal opening or lateral teeth. 

 Miller, however, figured a specimen of this shell in his North American Geology 

 and Paleontology, p. 460, with lateral teeth, and so at present the question 

 must remain over as to whether Ulrich's statement is correct. I find no large 

 byssal opening in the shells which I refer now to Ambonychia, and unfortunately 

 I can say little or nothing about the hinge teeth of most of them. A. triton 

 probably should be placed in a different genus when more is known about its 

 internal characters. Its broad subquadrate character resembles Gosslettia to some 

 extent. 



Mytilarca of Hall cannot be very far removed from Ambonychia, as is evidenced 

 by general shape and the posterior hinge teeth. Both genera, though mytiliform in 

 shape, have a relationship with the arciform Grammatodon and Palsearca in the 

 character of the posterior hinge teeth. 



The genus Ambonychia appears to be confined to Silurian rocks. 



Ambonychia triton, Salter, sp., 1848. (PI. I., figs. 26, 27.) 



Avicula triton, Salter, 1848, Mem. Geol. Surv. Gt. Brit., vol. ii., pt. i., p. 367, pi. xxiii. fig. 5. 

 Avicula {Ambonychia), Murchison, 1859, Siluria, 3rd ed., p. 213, fig. 8. 



Specific Characters. — Shell sub-rectangular, obliquely gibbose. The anterior end 

 obsolete, compressed, and subalate posteriorly. The anterior margin descends some- 

 what obliquely backwards ; lower margin short, slightly rounded, passing with a broadly 

 rounded curve into the posterior border, which is oblique and truncate, and makes 



