BRACHIOPODA — TELOTREMATA. 



333 



361. S. disjunctus Sowerby. (Fig. 424, b-d.) Devonic. 



Very variable in shape from semicircular to attenuate winged. 



Pedicle valve but slightly incurved at beak, its internal mold gives 



a triangular area in part marked by the muscular impression dis- 



Fig. 424. a, Spirifer mesicostalis ; b-d, Spirifer disjunctus showing variation of form ; 

 d, internal mold (Pal. N. Y.). 



joined from the sides of the shell by narrow fissures formerly occu- 

 pied by the dental lamellae. Surface of both valves, including sinus 

 and fold covered with very low, even plications. Sinus and fold 

 are sharply defined from the rest of the shell. 



Chemung; throughout North America.- Also Europe, etc. 



362. S. subattenuatus Hall. Devonic-Mississippic. 

 Small, with hinge line often extended into mucronate points. 



Mesial sinus deep and subangular. Surface with 8 to 12 strong 

 plications on each side the median line, crossed by sharp concentric 

 laminae. Differs from 5. mucronatiis in its smaller size, more ele- 

 vated plications and stronger imbricating lamellae. 



Chemung of New York and Marshall of Michigan; equivalent 

 horizons of Illinois, Iowa, Northwest Territory. 



363. S. keokuk Hall. (Fig. 425, a-b.) Mississippi. 

 Gibbous. Valves nearly equal in convexity. Pedicle valve with 



