486 DR WHEELTON HIND ON 



forwards, acute. Anterior surface depressed and hollowed, pierced below the beaks by 

 a circular aperture for the byssus. Hinge line straight, of moderate length. 



Interior. — Three short, diverging lateral teeth at the posterior end of the hinge 

 plate. Anterior adductor scar small, just within the umbones. Posterior adductor 

 scar large and shallow, placed low down and remote from the margin. 



Exterior. — The surface is ornamented with radiating ribs which increase in size and 

 strength as they pass across the valve. They are crossed occasionally by deep con- 

 centric lines of growth. 



Observations. — The presence of a large byssal aperture below the umbones separates 

 this genus from Ambonychia, with which it has, however, very close affinities. Ulrich 

 has described several species from the Trenton and Cincinnati groups of Minnesota and 

 Ohio. He states that he has found that, "within reasonable limits, the number of the 

 radiating costse is constant for each species, and the same in specimens of all ages." 

 On this point 1 am able to say nothing, as my material, though consisting of numerous 

 examples, is very imperfect. A large number of specimens show the byssal aperture, 

 and one the posterior part of the hinge plate. 



Byssopteria, Hall {Pal. N. York, vol. v., pt. i., p. 252), is probably either 

 identical or closely allied to Byssonychia, but as the anterior part of the shell was not 

 described it is impossible to make any definite statement at present. 



It is questionable whether the genus Byssonychia is really necessary. Hall 

 describes his genus Ambonychia as sinuate on the anterior side for the passage of the 

 byssus. 



Byssonychia sublsevis. (PI. I., figs. 25, 25a.) 



Specific Characters. — Shell small, triangular, oblique, moderately convex. The 

 anterior end obsolete, adpressed to form an anterior surface above with an acute border 

 below. The inferior border is narrow, rounded. The posterior border elongate, trun- 

 cate, gently curved. The hinge line short, straight. The umbones small, pointed, 

 incurved, terminal. 



Interior. — Unobserved. 



Exterior. — The surface is ornamented with numerous simple, radiating, fine striae 

 obsolete in the region of the umbones. 



Dimensions. — Fig. 25, PI. I., measures: dorso-ventrally, 18 mm.; antero- 

 posterior^, 10 mm. 



Localities. — Mulloch Hall, Lower Llandovery ; Woodland Point, Middle Llan- 

 dovery. 



Observations. — Two examples of this species are in the collection. One is a cast 

 of the interior, which unfortunately does not give any trustworthy details ; the other 

 shows the external ornament. Both specimens are crushed. The species is very much 

 smaller, and the radiating linear ornament much less pronounced than the strong ribs 

 which characterise B. radiata, for which it could not be mistaken. 



