BRACHIOPODA. 255 



centrally and notched or divided at its crest, making it bilobed or bidentate, 

 as seen from its inner surface ; on its outer or posterior face each apophysis is 

 deeply grooved, giving the entire cardinal process, from this point of view, a 

 distinctly quadrilobate appearance. The crural plates end more or less abruptly 

 in elevated points, which undoubtedly mark the place of attachment of the crura. 

 The bases of the crural plates are not produced around the muscular area. 

 Muscular impression Habelliform, very much as in the other valve ; divided 

 medially by a low faint ridge. Shell-substance strongly punctate. 



Type, Spirifer cre.nistria, Phillips. American example, Streptorhynchus Pan- 

 dora, Billings. Corniferous limestone. 



Observations. This group of shells is distinguished from the Silurian Stro- 

 phomenas by the following characters: the external form is subject to greater 

 variation and especially so in the .convexity of the valves ; the area of the 

 pedicle-valve is less constant, often higher, and not infrequently irregular or 

 unequal on the two sides ; the umbo is often distorted, and the apex seldom 

 if ever perforated by a small round foramen as is always the case in young, 

 and generally in mature individuals of Strophomena. The surface orna- 

 mentation is also coarser and more pronounced, and never so delicate as in 

 Strophomena planumbona, and its congeners. The cardinal process does not 

 merely rest upon a hinge-plate or umbonal callosity, but is distinctly coalesced 

 with the crural plates. In Strophomena these plates make a sweeping outward 

 curve and do not show the point of attachment of the crura, while in Ortho- 

 THETES their divergence is less, and they terminate abruptly in crural apophy- 

 ses ; the character of the muscular impressions is quite different. 



The group appears to be well-defined on the basis of these features, and 

 subject to little variation. As already observed, Orthothetes appears directly 

 upon the disappearance of the Strophomenas from the faunas of the Silurian 

 (Clinton); perhaps its earliest representatives, so far as known, are Strophomena 

 subplana, Conrad, and Streptorhynchus tenuis, Hall, of the Niagara group, followed 

 in the Lower Helderberg by Orthis deformis. Hall, Strophomena Woolworthana, 

 Hall ; becoming an abundant form in the Devonian, represented by Streptorhyn- 

 chus Pandora, Billings, Orthis prava, Hall, Strophomena Chemungensis, Conrad, and its 



