32 PALAEONTOLOGY OP THE UPPER MISSOURI. 



sures.* It also occurs in the Permian rocks, and may possibly range up into the 

 Trias, 



JHyaliiia perattenuata. 



(Plate I, Fig. 12, a, b.) 

 Myalina perattemiata, Meek & Hayden, Trans. Albany Institute, IV, Marcli 2, 1858. 



Shell very thin and fragile, obliquely elongate, narrow and slightly arcuate ; valves convex anteriorly, and com- 

 pressed behind. Beaks pointed, terminal and attenuate ; hinge line equalling rather more than lialf the entire 

 length of the shell, and ranging at an angle of about 50O above the oblique anterior margin. Posterior border 

 sloping from the end of the hinge, nearly parallel to the anterior side above, and rounding to the narrow antero- 

 basal extremity below ; anterior margin of the valves a little arcuate, and rather abruptly deflected inwards from 

 the umbonal ridge above the middle, and in outline nearly straight below. Umbonal slopes prominent from the 

 beaks down tlie anterior side. Surface with obscure subimbricating marks of growth. 



Length from the beaks to the postero-basal extremity, 1.50 inch ; breadth, 0.65 inch; convexity, about 0.44 inch. 



This species will be recognized by its slender attenuate form, and very thin shell. 

 Like other species of the genus, it seems to have varied more or less in form, a 

 portion of the specimens being straighter on the anterior margin than others. 

 We know of no species with which it is liable to be confounded. Mi/alina (Modiola) 

 minor, Lea (Jour. Acad. Nat. Sci. Phila., Vol. II, 2d ser., p. 205) seems to be an 

 analogous species, but is more finely and regidarly striated. 



Locality and position. — Opposite the northern boundary of Missouri, on the 

 Missouri River. From seams of yellowish magnesian limestone, in the Coal 

 Measures. (Type 1022a.) 



iTIyalina siibqnadrafa. 



Myalina subquadrata, Siicmaed, Geol. Report Missouri, part Palffiontology, p. 207, pi. C, fig. 17, o, b. 



Shell attaining a large size, compressed and somewhat alate in the postero-dorsal region, and convex anteriorly — 

 considerably higher than long ; posterior margin forming a broad gentle curve, being nearly straight and ranging 

 almost vertically near the middle, and curving forward so as to intersect the hinge at an obtuse, undefined angle 

 above, while below it arches regularly forward into the rather narrowly rounded base ; anterior margin thickened 

 within above, broadly arcuate or concave in outline, its curvature being nearly parallel to that of the posterior 

 margin. Beaks terminal, directed forward ; umbonal ridge most promiuent and oblique above, and in adult shells 

 curving downwards so as to range nearly vertically near the middle. Hinge line straight, and ranging nearly at 

 right angles to the longer, or vertical axis of the valves ; cartilage furrows distinct, straight, and in mature shells 

 numbering about ten or twelve ; area broad. (Muscular impressions unknown.) 



Height about 4 inches ; antero-posterior diameter at the middle, 2.40 inches ; convexity of a left valve, 0.83 inch. 



' Prof. Hall refers an Upper Silurian shell, from the Clinton Gronp (Pal. N. Y. Vol. II, p. 100, 

 pi. xxx), to this genus ; but as his specimen is an imperfect cast, its true relations remain doubtful. 



