135 



There is a scarcity of growth cicatrices on the valves of this species ; 

 seldom are more than two seen on the ventral, and on the dorsal, one or 

 none. This may be due to the advanced position of the muscle scars and 

 the thickness of the shell. 



OBOLUS, Eichwald. 



" In default of more exact criteria, we have adopted here and elsewhere Genus Obolus 

 the following characters' for distinguishing Obolus from Lingulella, viz: 

 roundness of outline, short cardinal area and depressed beak?, advanced 

 position of muscle scars in the valves, and strong arch of the vascular 

 trunks in the ventral as well as the dorsal valve. (Sub-genus Palseobolus 

 is an exception.) There is, however, a more important distinction, which, 

 in consequence of imperfect preservation of the valves, can seldom be 

 observed, that is, the position of the secondary muscles of the central 

 group in the ventral valve, as compared with the great muscle of that group. 

 In Obolus they are lateral, but in Lingulella anterior to the great muscle. 

 This shows a radical difference of structure between the two genera." 

 [Bull. Nat. Hist. Soc. N.B,. vol. iv, page 198.] 



There is also an important distinction in the advanced position of the 

 scars of the " j " laterals in the dorsal valve of Obolus ; in this genus 

 they are placed toward the front of the valve, but in Lingulella they are 

 not far removed from the centrals. As described by tte Russian writers 

 Obolus (sens, strict.) appears to range from the Dictyonema zone upward ; 

 it is, therefore, properly an Ordovician genus, while Liugulella is numerous 

 in the Cambrian. For practical purposes (owing to difficulty in finding 

 valvess howing the internal markings distinctly), it is convenient to use the 

 diagnosis of Obolus given above, while at the same time it must be 

 acknowledged that it is an unsatisfactory one. And the more so because 

 wherever we have been able to tind the internal markings of the Etche- 

 minian Oboli they do not agree with the typical interior of Oboli 

 obtained on the shores of the gulf of Finland. In this way we have 

 come to recognize the fact that these Canadian Oboli, though similar in 

 the form of the valves to the later ones of Russia, have had different 

 origin. Of the two types that occur in the Etcheminian terrane, the 

 older one in the arrangement of the central group of scars of the dorsal 

 differs more from Obolus than that from Lingulella, and we have thought 



it necessary to establish a subgeneric distinction, as follow : 







EOOBOLUS, n. subgenus. 



In Obolus proper the anterior adductors of the dorsal valve are far 

 apart and have a position at the mid length of the valve, while the " j " sub-genus. 



