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 B, FIGS. 1-6. 
1S62. Trimerella, Billings. Palaeozoic Fossils, vol. i, p. 166. 
1867. Trimerella, Lindstrom. Ofvers. af Kongl. Vetenskaps-Akad. Forhandl. No. 5, p. 253, pi. xxi. 
1868. Trimerella, Lindstrom. Geological Magazine, vol. v, p. 441. 
1870. Gotlandia, Trimerella, Dall. American Journal of Conchology, vol. ii, pt. 2, p. 160. 
1871. Tnmerella, Meek. American Journal of Science, Third Ser., vol. i, p. 305. 
1871. Trimerrella, Dall. American Journal of Conchology, vol. vii, pt. 2, p. 83. 
1872. Trimerella, Davidson and King. Report Brighton Meeting Brit. Assoc. 
1873. Trimerella, Hall. Twenty-third Rept. N. Y. State Cab. Nat. Hist., pi. xiii, figs. 11-16. 
1873. Trimerella, Meek. Geology of Ohio; Palaeontology, vol. i, pp. 182, 183, pi. xvi. 
1874. Trimerella, Davidson and King. Quart. Journ. Geol. Soc., vol. xxx, p. 143, et seq. 
1875. Trimerella, Nicholson. Rept. Palaeont. 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. 
