2.'i7 



111 the absence of the muscular markings and loop, it has been 

 impossible to determine Avith accnracy whethei" this species is a true 

 Terehratula or not, since, so far as external form is concerned, it 

 might belong either to CentroneUa Billings, or to Cryptonella Hall. 

 Until more perfect material shall have been collected, I have 

 thoiight it best to refer the species provisionally to Terehratula, 

 tht? most largely represented by species of the above genera. (Mor- 

 gan Expeditions 1870 and '71.) 



Named by Prof. Hartt in honor of his assistant, Mr. 0. A. Derl)y, 

 instructor in Geology and Palaeontology in Cornell University, and 

 his companion on two expeditions to the Amazouas. 



Spiril'cra Tcdroana, IIartt,sp. nov., Plate VIII, figs. 1-9, 13, U ami 10-20. 



Test of moderate size, inequivalve,very transverse, tliiu. Breadth varying- 

 from twice, to three and a half times the length, being greatest along the hinge 

 line. Outline sub-semi-elliptical or broadly' sub-triangular, the lateral margins 

 on each side forming a single, more or less strong, regular curve, though they 

 are sometimes nearly straight. Cardinal extremities more or less produced 

 and angular, varying from quite acute to nearly rectangular, often slightly 

 rounded. Test plicate. 



Ventral valve much more convex than the dorsal, sub pyramidal when 

 young, more or less ventricose when old. Greatest elevation at or just in 

 front of the beak, which is small, elevated, generally slightly incurved, but 

 sometimes hardly produced beyond the hinge area. Hinge area moderately 

 broad, triangular, nearly flat or slightly concave, perpendicular to antero-pos- 

 terior diameter or slightly inclined forwards or backwards, in which last case 

 it is generally slightly concave, the curvature varying somewhat but always 

 more marked under the beak. Cardinal margins angular, nearly straight or 

 curving very slightly inwards. Fissure triangular with the width at base 

 about equal to the height. Mesial sinus of moderate depth and width, broader 

 than deep and increasing gradually in size towards the front, where it is 

 slightly produced beyond the margin of the valve. It is regularly rounded in 

 the bottom, though sometimes slightly flattened towards the front ; its surface 

 is smooth and the margins are well defined. From the beak to the front, along 

 the mesial line, the surface of the valve curves moderately and regularly, but 

 never very strongly ; sometimes it is nearly straight. The slope from the 

 margins of the sinus to the cardinal extremities is very slightly convex but 

 often nearly straight. 



Dorsal valve moderately convex but sometimes much depressed, the eleva- 

 tion being greatest near the middle. Beak minute. Mesial fold prominent and 

 abrupt, moderately wide, its breadth increasing regularly from the beak to the 



