part 4] CAMBRIAN HORIZONS OF COMLET. 355 



The wrinkles of the outer surface recall those of II. corrugatus 

 Kiser l ; they are, however, more irregular and less coarse than 

 those shown in Dr. Kiser's figure. He regards his species as 

 belonging to the sub-genus Orthotheca. 



The resemblance in the shell-structure of this species to that of 

 H. sculptilis, sp. nov. (p. 356) is very close, although the character 

 of the sculpture differs radically. 



Locality and horizon. — Comley, Lower Cambrian ; from the 

 Protolenus Limestone, Ac 5 . 



Hyolithus strettonensis, sp. nov. (PL XXIV, rigs. 30 & 31.) 



Hyolithus cf. primcevus Groom, Cobbold, 1916, Rep. Brit. Assoc. (Manchester, 



1915) p. 118. 

 Cf. Hyolithus primcevus Groom, 1902, Q. J. G. S. vol. lviii, p. 116. 



Type-specimen [1903]. 



Diagnosis. — Shell straight, tapering uniformly at rates which 

 vary between 1 in 4 - 5 (apical angle 13°) and 1 in 7 (apical angle 8°); 

 section a segment rather greater than a semicircle, ratio of dia- 

 meters about 1 to '66, dorsal side very slightty convex, ventral 

 semicircular, lateral angles rounded ; aperture in two planes, 

 projection of dorsal lip about "2 of the width ; sculpture consisting 

 of faint closely-set striae of growth. 



Dimensions. — Width of aperture = 6 to 7 mm. ; total length 

 = 30 to 40 mm. 



Hyolithus strettonensis, var. brevis nov. (PI. XXIV, 

 fig. 32.) 



Prom the same general horizon a variety, or allied species, is 

 found, which has a rate of taper varying from 1 in 2 - 5 to 1 in 3"5 

 (apical angles 23° to 18°), and a rather smaller proportional 

 projection of the dorsal lip. 



Dimensions. — Width=about 55 mm. ; length = 16 mm. 



The variation in the apical angle is similar to that of H. ameri- 

 canus Billings, 2 which differs from H. strettoiiensis in having a 

 greater proportional projection of the dorsal lip and a triangular 

 section. 



H. communis Billings 3 also has a greater projection of dorsal 

 lip than //. strettoiiensis ; it is more elongate, and its section is 

 flattened both dorsally and ventrally. 



A curious confusion appears to have crept in as to the sub- 

 generic reference of H. communis. In the original description 

 Billings distinctly refers to the projecting [dorsal] lip ; he, however, 

 describes an operculum under the same reference, which seems to 

 belong to an Orthotheca. Walcott (1886, p. 136) adopts Billings's 

 description, and then observes that Ford's species H. emmousi 

 seems to be a variety. From the original description (Ford, 1873, 

 p. 214) this species is evidently an Orthotheca, and is recognized 



1 1916, p. 25 & pi. iii, figs. 10a-10 6. 



- 1872, p. 215, text-fig-s. 2 a & 2 b :! 1872, p. 214. 



