41 



of variable size usually 6 or 8 in 1 line, but in some places only 3 or 4. 

 These are crossed by small threadlike radiating ridges of which there are 

 5 or 6 in the width of 2 lines about the middle of the shell. 



Length 2 inches ; width about the middle 1^ inches. 



This species is allied to A. elliptica (Hall) but differs in being more 

 convex and pointed above and also in having the surface reticulated by 

 both radiating and concentric striae. 



It seems to be very rare, only one imperfect specimen having been 

 found. 



Locality and Formation. — Montreal. Trenton limestone. 



Collector. — E. Billings. 



CONOCARDIUM IMMATURUM. (N. Sp.) 



Fig. 43. 

 Fig. 43. — Conocardium immaturum, enlarged. 



Description. — This little species is from 2i to 4 lines in width on the 

 hinge-line and li to 2i in length measured from the umbones to the ven- 

 tral margin. The body of the shell is subtriangular, ventricose, the ven- 

 tral margin rounded, the posterior angle about one fifth more remote from 

 the umbones than the anterior, the umbonial angle acute, apparently 20° 

 or 30°. The anterior wing is triangular compressed, its dorsal margin 

 convex, forming with the anterior margin an angle of about 45°. At the 

 junction of the lower margin of the wing with the ventral margin of the 

 body of the shell there is an obtuse notch, in consequence of the curvature 

 of the two margins mentioned. The posterior wiag is small slender and 

 cyUndrical at its extremity but towards the body of the shell it rapidly 

 expands into a broad base occupying about i the length of the posterior 

 side of the body of the shell. The lunbones are small but distinctly eleva- 

 ted above the hinge line. The beaks are closely iucurved. 



Surface covered with fine radiating ribs, just visible to the naked eye. 

 Of these, 10 can be counted on the anterior wing and about double the 

 same number on the body. 



The following are the dimensions of the most perfect specimen observed. 

 Width on hinge line including both wings 2i lines ; from the umbones to 

 extremity of anterior wing 1^ lines ; from the umbones to extremity of 

 posterior wing 1 line ; from umbones to ventral margin IJ lines ; width 



D 



