BRACHIOPODA. 43 



Generally these muscular scars are sharply defined. Those on the platform 

 of the pedicle-valve indicate very large organs, but in M. prisca we have 

 been unable to resolve them as Davidson and King have done. In M. Greenii, 

 the lateral scars are clearly outlined and limited to a comparatively small sur- 

 face. Their small size may be largely due to the fact that the valves of this 

 species were comparatively light ; their position, close together on either side 

 of the median septum, is very suggestive of the muscular arrangement in 

 ScHizocRANiA In M. Ortoni there appears a prominent anterior scar with fainter 

 laterals, bordered in front by narrow converging furrows. 



On the brachial platform, the subdivision of the muscular impression is 

 always strong, producing a trilobate appearance ; sometimes the shell is so 

 preserved as to show the long progressive track of the laterals and the centrals, 

 at others, the older portions of the scars are covered by an organic deposit 

 limiting the impressions to the area last covered by the muscular bands. 



In one species, M. Ortoni, the impressions of the pallial vessels, both primary 

 and secondary, are very distinct, and this is the only instance in which the 

 ramifications from the main vascular trunks have been observed in this group. 



MoNOMERELLA is readily distinguished from the allied genera by its two umbonal 

 chambers and low, unvaulted platforms. In the brachial valves of this genus, 

 and of DiNOBOLUS and Rhinobolus, the similarity in internal features is very 

 close, and care is required in the generic determination. In Dinobolus the plat- 

 forms are always vaulted, to some degree, by conical cavities, and not merely 

 broadly excavated on the anterior walls ; the umbonal cavity is a small, acute 

 depression, never filled by testaceous deposit ; the crescent is always broad, with 

 abrupt lateral angles. Rhinobolus possesses a very short platform, strongly 

 elevated at its anterior margin, and sloping abruptly backward, but there is no 

 evidence of its general suppression as sometimes occurs in Monomerella. 



. Species of this genus, _ like those of Trimerella, are very limited in faunal 

 range. The Canadian examples are all from the Guelph limestones of the 

 Province of Ontario ; those from the States of Ohio, Illinois and Wisconsin 

 are from dolomites usually referred to the age of the Niagara of New York ; 



