MUSEUM OF COMPAKATIVE ZOOLOGY. 33 



tlie ornamentation of the surface cannot be determined from the specimens at 

 hand. Mr. Billings describes them as being finely striated, "the stride curving 

 forwards on the dorsal side, then passing upwards on the sides at nearly a right 

 angle, curve slightly backwards on the ventrum." The specimen described 

 here is 11 mm. long, and is one of the smaller specimens of the species. 



Locality and position. — Station No. 2, North Attleborough, Mass., Cam- 

 brian, one specimen. It occurs also at Troy, N. Y., and at Bic and St. 

 Simon in Canada. 



14. Hyolithes princeps, Billings. 



Plate II. Fig 25. 



Shell large, straight, gradually tapering, very thin. The shells occur in 

 large numbers throughout the section at Locality No. 1. Their outline is 

 readily seen on almost any fracture of the rock in the bed in which the species 

 is found ; but owing to the irregular fracture of the rock and the large size of 

 the species, as well as to the very frail nature of the shell, entire specimens 

 are not found. The following description is dxawTi up from abundant frag- 

 mentary material. 



The shells vary greatly in size, reaching at maturity a diameter of 9 or even 

 11 mm. and a length of 100 mm. or more. The dorsal side is flattened or mod- 

 erately convex. The ventral side is decidedly convex; usually the convex- 

 ity is more marked on one side of the ventral surface than on the other, the 

 latter side being often almost flat. The median line is rounded, yet usually 

 distinct enough to be recognized. The apical angle is very moderate. In some 

 specimens it is as low as 6°. In the original description of the species, it is 

 said to be as high as 15°. The lateral angles are also more rounded than in 

 typical specimens. The surface is marked by fine transverse strife. 



It will be noticed that the specimens here described do not closely accord 

 with the type. The lateral angles are not prominent enough, and the apical 

 angle is lower; but they agree with those forms in size and in general appear- 

 ance. At any rate, the amount of variation seems insufficient to be ranked as 

 specific. One interesting feature of these specimens is the frequency with 

 which the shells of different individuals are found loosely inserted in each 

 other so that three or four shells are successively sheathed one within the 

 other, or they may be inserted side by side in a large individual. Small 

 slender shells apparently belonging to Hijolithellus micans also occur in this 

 position. Whatever may be the conditions of this sheathing, it does not 

 suggest any structural connection between the difl'erent shells at the time of 

 fossil ization. 



Locality and position. — Station No. 1, North Attleborough, Mass., Cam- 

 brian, 200 specimens. 



VOL. XVI. — NO. 2, 3 



