BRACHIOPODA. 33 



and, usually, anterior scars; the post-median impression, indicated upon David- 

 son and King's figure above given, and considered by these authors as the 

 marks of the genitalia, is, usually, extremely indistinct. Just within the 

 cardinal margin are seen, more clearly in Rhinobolus than elsewhere, two small, 

 distinct scars (umbo-laterals), the function and analogues of which are not 

 understood. 



Genus TRIMERELLA, Billings. 1862. 



PLATE IV A, FIGS. 1-10; and PLATE IV E, FIGS. 1-6. 



1862. Trimerella, Billings. Palteozoic Fossils, vol. i, p. 166. 



1867. Tvimerella, Lindstrom. Ofvers. af Kongl. Vetenskaps-Akad. Foi-handl. No. 5, p. 253, pi. xxi. 



1868. Trimerella, Lindstrom. Geological Magazine, vol. v, p. 441. 



1870. Gotlandia, Trimerella, Dall. Amei-ican Journal of Conchology, vol. ii, pt. 2, p. 160. 



1871. Trimerella, Meek. American Journal of Science, Third Ser., vol. i, p. 305. 



1871. Trima-ella, Dall. American Journal of Conchology, vol. vii, pt. 2, p. 83. 



1872. Trimerella, Davidson and King. Repoi-t Brighton Meeting Brit. Assoc. 



1873. Tnmerella, Hall. Twenty-third Rept. N. Y. State Cab. Nat. Hist., pi. xiii, figs. 11-16. 



1873. Trimerella, Meek. Geology of Ohio; Palseontology, vol. i, pp, 182, 183, pi. xvi. 



1874. Trimerella, Davidson and King. Quart. Journ. Geol. Sec, vol. xxx, p. 143, et seq. 



1875. Trimerella, Nicholson. Rept. Palaont. Prov. Ontario, pp. 67, 68, figs. 36, 37. 



Diagnosis. Shell thick; outline elongate-ovate. 



Pedicle-valve with an erect and straight umbo, which is usually solid for the 

 greater part of its length, but double-chambered toward its base ; cardinal area 

 very prominent ; deltidium broad ; deltidial ridges more or less conspicuous ; 

 deltidial callosities slightly developed; cardinal slope well defined. Crescent 

 sharply incised over its summit, but faint toward the sides, where it is con- 

 nected with the broad lateral and terminal scars which are not so distinctly 

 outlined as in the allied genera. Platform long, narrow, and strongly devel- 

 oped ; platform-vaults tubular, the dividing wall being continued as a short 

 septum in front of the platform. Muscular scars of the platform usually ex- 

 tremely faint, but under favorable preservation divisible into median and lateral 

 components. Pallial sinuses faint. 



Brachial valve with a marginal umbo, which is much thickened in some 

 instances, without producing a cardinal area, though making a prominent 

 apophysis which is extended against the cardinal slope of the opposite valve. 



