ILLUSTEATIONS OP THE FAUNA OF THE ST. JOHN GEOUP. 103 



A partly decorticated example of the dorsal valve, exhibits a small bifid scar at the 

 hinge-line, due to the adductor muscle. There is no median ridge, such as is found in 

 Orthis, the median sinus being almost obsolete. In front of the impression of the ad- 

 ductor muscle is a small, low callus. 



Sculpture. This consists of sharply raised, rounded, radiating ridges, slightly crenu- 

 lated by obscure, transverse lines. These ribs are of varying size, every third or fifth rib 

 being larger than the others ; these large ribs extend outward from the umbo, and the 

 smaller ones are intercalated between them, or spring- from them. On the dorsal valve 

 there are two main ribs in the sinus of the valve and three others extending to the 

 lateral margins ; smaller ribs are intercalated between these. On each side of the group 

 of ribs on the middle third of the shell are four fainter, but more strongly arched groups 

 of ridges which throw off minor ridges at the back, directed toward the lateral margins. 

 On the ventral valve the small ribs are almost all intercalated and do not spring from the 

 back of the primary ribs, as in those of the dorsal valve. 



Size, Length, 12 mm. Width 14 mm. Height of area of dorsal valve | mm , of vent- 

 ral valve 1 ram. A valve supposed to be of this species is 15 mm. long and 19 mm. 

 wide. 



Horizon and Locality. In dark shales of division Sa at Navy Island. 



One of the oldest species of Strophomena hitherto known is S. aurora, Billings, from 

 the Quebec G-roup on the west coast of Newfoundland.' This is said to range from strata 

 equivalent to those of the Chazy group into those of the Levis slate. It is, therefore, nearly, 

 but not quite as old as our species. The form (outline) as figured by Billings, is that of a 

 much wider shell, but as he gives a wide range of variation in the width it is evident 

 that some approach very closely to the St. John species. In both species the striœ are 

 two ranked, ie., some much larger than the others, but in Billings's species the striœ are 

 said to be angular, while in the St. John species they are rounded. Another point of dif- 

 ference between the two is in the umbo of the ventral A'alve, which in S. aurora is said to 

 be often acutely carinated, but in S. atava is fiat and inconspicuous. The hinge area also, 

 in both valves of the latter species, is lower than in the former. In the markings of the 

 interior of the ventral valve and in the form of the ribs on the surface of the shell this 

 species resembles S. a/to-rtato, Emmons," of the Trenton and Hudson River groups. In 

 these three species we seem to have a genetic series, with progressively larger size and 

 higher hinge area. 



CAMERELLA, Billings. 



Camerella parvA, Bill. ? (PI. VII., figs. 9 a and b.) 



Palaeozoic Fossils, vol. i., p. 210. 



A small species of Camerella occurs in Division 3, which, so far as the incomplete 

 valves show, appears to agree with Billings's species above named. It is sub-triangular in 

 form with a shallow triangular sinus. The decorticated beak exposes some of the internal 

 inequalities of the surface of the shell. It has a narrow, lanceolate, bifid muscular im- 

 pression due to the adductor muscle, and on each side of it a club-shaped impression, pro- 



' Palaeozoic Fossils, vol. i., p. 218. 



- Report State Geol. New York, 18S2. PI. (viii.) 38 — also Geol. Surv. Ohio, Palaeontology, vol. i., PI. 7. 



