560 GEOLOGY OF OHIO. 



Mr. E. O. Ulrich, to whom this specimen was referred for generic 

 determination, says regarding the same: "The lamellibranch I regard 

 as a species of Mytilarca which is placed with the Ambonychiid^ in the 

 new classification. Hall's Ambonychia aphara, and acutirostra are 

 regarded as congeneric, likewise his Myalina mytiUformis. All four 

 of these species are closely related but it can not be said that 

 any two are identical. They have cardinal and obscure lateral teeth as 

 in Ambonychia radio ta, a species really distinct from Ambonychia. Myti- 

 larca and Plethomytilus differ chiefly in havTttgja 'byssal opening. In this 

 they agree on the other hand with the original Ambonychia, which differs 

 again in having radiating striae and a peculiar plate beneath the beaks." 



Modiolopsis rkomboidea, Hall. 



(Plate 37. Figs. S a. 



In the upper shaly courses of the Clinton at Huffman's Quarry the 

 interior cast of a shell was found presenting the following characteristics : 

 It is 20 mm. long. The form is rhomboidal oval; the greatest height is 

 at the posterior end of the hinge line, where it is about 11.3 mm. The 

 length of the hinge line posterior to the beak is 12 mm. The posterior 

 end of the shell is 15.5 mm. distant from a line drawn vertically through 

 the beak. The hinge line in the cast leaves an elevated line both before 

 and after the beak. The shell was but slightly convex except along the 

 umbonal ridge. In the cast a line of depression or broad groove extends 

 along, and a short distance within, the basal margin; it becomes more dis- 

 tant from the margin going posteriorly, also at the anterior extremity of 

 the cast, where this groove rounds towards the beak. Anterior to this 

 curved line' at the anterior end of the cast is a semi-lunate elevation, cor- 

 responding to a similar depression in the original shell. These markings 

 indicate the presence of a raised pallial line, and of a deep anterior muscu- 

 lar scar in the original. The identity of this specimen with the species 

 described by Hall from the Upper Silurian at Arisaig, Nova Scotia, is 

 probable. (Plate 37, Fig. 8 b.) 



Modiolopsis subrhomboidca, Simpson. 



Plate 37, Figs. 7 a. 



At Huffman's Quarry in the upper shaly courses was found a valve 

 17 mm. long and 9.5 mm. high. The basal line is moderately convex, 

 curving strongly at its extremities. The anterior margin is strongly 

 rounded, being concave however just before the beak. The posterior mar- 

 gin is slightly convex, or quite straight, meeting the hinge line at an angle 

 of about 125 degrees. Valves moderately convex, more strongly convex 

 along the umbonal ridge. The hinge line is slightly oblique. The 

 posterior end of the hinge line is about 10 mm. from the beak. The 



