378 THE PALEONTOLOGY OF MINNESOTA. 



[Clitambonites diversa. 



"Type: Pronites adscendens Pander." (Hall, op. cit.) 



In well-preserved specimens of C. diversa the upper surface of the spondylium 

 is transversely striated, these striae having three distinct curvatures in passing over 

 it. Since their position and the area occupied agree with the muscular scars of this 

 valve in Orthis, they are here regarded as homologous with the adductors, diductors 

 and adjusters of that genus. In Lingulelasma, Lingnlops and the trimerellids the mus- 

 cular scars are not found in front nor underneath, but on the "platform" of those 

 genera. The platform, therefore, is homologous with the spondylium of Clitambon- 

 ites and Pentamerus. In Pentamerus galeatus Dalman, of the Lower Helderberg group 

 of New York,* the adductors and diductors occupy nearly the entire spondylium, 

 while the adjusters were probably situated on narrow flanges of the walls of the 

 delthyrium. The portion of the valve immediately beneath the spondylium, and 

 occasionally the sides of the septum, are strongly marked by the genital sinuses. 

 Since there is no space posterior to these markings for the attachment of the 

 muscles, this clearly indicates that they were situated on the upper surface of the 

 spondylium. 



CLITAMBONITES DIVERSA Shaler, tt/i. 



PLATE XXX. FIGS. 11-17. 



1865. Orthisina diversa SHALER. Bulletin of the Museum of Comparative Zoology, no. 4, p. 67. 



1866. Orthisina verneuili BILLINGS. Catalogue of the Silurian Fossils of Anticosti, pp. 43, 74. 

 1877. Hemipronites americanus WHITFIELD. Annual Report of the Geological Survey of Wisconsin, 



p. 72. 



1882. Hemipronites americanus WHITFIELD. Geology of Wisconsin, vol. iv, p. 243, pi. x, flgs. 15-17. 

 1889. Streptorhynchus americanus MILLER. North American Geology and Palaeontology, p. 378. 

 1892. Clitambonites americanus HALL. Palaeontology of New York, vol. viii, p. 239, pi. xvA, flgs. 1-8. 



Original description: " Toothed [ventral] valve, usually pentagonal; socket- valve 

 quadrate; hinge-line usually equal to the greatest width of the shell. Toothed valve 

 very strongly projecting; depth about one-half the width; deepest point about the 

 hightof hinge-line; umbo somewhat laterally compressed, usually rising high above 

 the plane of the hinge-line, but very variable in this respect; umbo always laterally 

 inclined indifferently towards either extremity of the hinge-line. Surface near the 

 extremities of the hinge-line a little depressed and slightly recurved; area very large, 

 nearly half as wide as long. Fissure from one-fourth to one third the width of the 

 hinge-line; deltidium large, massive, rarely central, with distinct circular or oval 

 foramen. Socket-valve with a broad and shallow mesial fold." 



Shell of medium size; subquadrangular in outline; hinge-line straight, rarely 

 shorter, and usually as long as, or slightly longer, than the greatest width of the 

 shell; cardinal angles often mucronate; lateral margins straight or nearly so, sloping 



*Pal. New York, vol. Ill, p. 257, pi. xi/vi and XLVII, 1859. 



