44 



Fossils of the Loiver Silurian Rods of Canada. 



whorls touch each other, but in those which are badly preserved, they are 

 separate, as shewu in the above jQgure. This very interesting and often 

 beautiful fossil is not found in neither the Trenton Limestone or Utica Slate, 

 being confined to the Hudson Kiver group. Specimens have been procured 

 at Toronto. In the Trenton Limestone there are several other species of 

 this genus also very beautiful in their form and sculpture." 



The generic name is from the Greek, Kurtos, curved ; and Liihos, 

 stone. Ornatus, Latin, ornamented. 



Ambonychia radiata is one of the most common and characteristic 

 fossils of the Hudson River group. In the system of classification given on 

 page 31 , this and the next following species would rank among the AcepJi- 

 ala or headless mollusks, of which the common clam-shells of our rivers and 

 lakes are members. Fig 7 is the usual form, although it is frequently much 

 smaller, and not so acute above. The surface is marked by from twenty- 

 five to forty strong radiating ridges which are somewhat flattened upon the 

 top and crossed by fine concentric striae. The grooves between are rounded 

 on the bottom, and half the width of the ridges. 



The name Ambonychia is from the Greek Ambon, the boss of a shield, 

 and Onyx, a claw in allusion to the rounded and claw shaped beak of some 

 of the species, " Radiata,'' radiated. This fossil is abundant m the Hudson 

 River group, but is not found in any other formation. It was originally 

 called Pterinea carinata, and is often quoted by that name in different 

 works. 



Fig. 8. — Modiolopsis modioloris. 



This fossil abounds in the Hudson River group, being characteristic of 

 the central and higher portions of the formation. It is of an exceedingly 

 variable form, and is thus described by Professor Hall : " Somewhat 

 obliquely oblong-ovate, narrowed before, expanded and obliquely truncated 

 posteriorly, basal margin usually contracted or slightly arched upwards ; 

 cardinal line extended straight, or slightly curved ; beaks moderately prom- 

 inent near the anterior extremity ; an oblique scarcely defined ridge, 

 extending from the posterior basal margin ; surface marked by concentric 

 undulations ; muscular impression distinct close to the anterior extremity." 

 In the above figure the narrow end on the left is the anterior, and the other 

 the posterior extremity of the shell. In the living animal, the head and 

 mouth occupied the small end, and hence it is called anterior. 



Prof Hall further states, that, "the fossil presents considerable 

 variation in form, which has given rise to the establishment of several 

 species, founded either upon natural or accidental characters. The 



