[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 115 



break along the lateral angles ; when the sides are thus separated frag- 

 ments ma}' be mistaken for the pleur£e or spines of tiilobites. 



Though the central depression of the dorsal side of the species is 

 more than two-thirds of the whole width of that side, the species may be 

 regarded as falling within the section Plicati of G. Holm and in this sec- 

 tion under the grouj? A6., typified by 0. affinis, to which, however, the 

 preceding S2)ecies is more nearly related than this one is. 



This species differs from Hyolithes Americanus in the absence of a lip 

 to the dorsal side, and in having the lateral angles more acute than the 

 median angle of the ventral side. It differs from all the Orthothecse 

 described by Walcott (under Hyolithes) in its angular form. It differs 

 from all those described by Gerhard Holm by the acuteness of the lateral 

 angles, and by the concave form of the ventral slopes near those angles. 



HYOLITHES, Eichwald. 



Hyolithes excellens, Bill. (PI. VII., figs. 3a to i.) 



Hyolithes excellens, Billings, PaUieozoic Fossils, vol. ii., pt. i, p. 70, fig. Sgr. 

 Hyolithes excellens. Bill., Nat. Hist. Soc. N.B., Bull, xviii., p. 194, pt. iii., figs. 3a to i. 



This is the leading species of the Etcheminian Fauna at Smith Sound, 

 being the largest and most abundant one. 



The following is Billings's description : " Shell usually about two 

 inches in length, tapering at the rate of between four and five lines to the 

 inch. The ventral [dorsal] side is nearly flat, or very gently convex ; 

 the lateral edges narrowly rounded, in some specimens rounded angular ; 

 the most projecting parts of the sides are about one-third the height ; 

 above this the sides are gently convex, the dorsum more narrowly rounded. 

 The shell is thin, nearly smooth, with very fine obscure striœ, about ten in 

 one line. The striae curve forward on the ventral [dorsal] side, forming 

 an arch, the height of which is equal to about one-third the width of the 

 shell. On crossing the lateral edges the striœ curve backwai'ds until they 

 reach the most projecting part of the sides, then cross up and over the 

 dorsum [venter] at a right angle. On a side view the shell is gently 

 curved downwards on approaching the apex." 



" A specimen twenty-four lines in length on the ventral [dorsal] side 

 in eight and one-half lines wide and six in depth at twenty lines from 

 the apex." 



" A specimen twenty-four lines in length on the ventral [dorsal] side 

 is eight and one half lines wide and six lines in depth at twenty lines from 

 the apex." 



" Occurs in red limestone at Smith's Sound, Trinity Bay." 



The extreme length of this species would be about 54 mm., judging 

 from the rate of tapering ; but all adult shells found were decollated. 



