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Published online by Cambridge University Press: 17 March 2016
The shells composing this species vary but slightly in shape, being marginally semicircular, concavo-convex, and about one-third wider than long. The hingeline is straight, and either a little shorter, with its cardinal angles rounded, or somewhat longer than the greatest width of the shell, with rectangular or slightly acute and extended terminations. Both valves are provided with narrow sub-parallel areas, the ventral one, which is the largest, being divided by a small fissure, partially covered with a pseudo-deltidium; while in the middle of the ventral one there exists a prominent V-shaped cardinal process. The ventral valve is moderately convex, and flattened towards its auriculate cardinal extremities. The beak, which is small and incurved, does not overlie the hinge-line; while the dorsal valve assumes in different specimens a greater or lesser degree of concavity, and follows the curves of the opposite one. Exteriorly the surface of the ventral valve is covered with numerous small thread-like radiating striæ, which increase in number by occasional bifurcation, or interstriations at various distances from the beak, so that as many as one hundred and twenty ribs may be counted round the margin of certain specimens, while at irregular distances small spines projected from the rounded surface of the striæ. In addition to these, on each side of the beak there exists along the cardinal edge from five to nine slanting tabular spines, which become longer and larger as they approach the extremities of the cardinal edge. The surface of the dorsal valve is striated as in the ventral one; and minute perforations or punctures may be perceived over the entire surface of the shell, and which are the exterior orifices of the canals which traverse the shell, as in Productus.
page 219 note * Geol. Survey of Canada. Report for 1855.
page 219 note † Geol. Survey of Canada. Eeport for 1857, p. 154.
page 225 note * In Ireland C. quadrata is found at Rahan's Bay, in Donegal. Appears to be a rare shell in English Carboniferous strata, a single example having been hitherto found by Mr. C. Moore, at Holwell, near Frome. It occurs also at Toumay, in Belgium, and at Tuscom bia, Alabama, in America. It may likewise be observed that many species of Crania have been confounded with Discina, from the fact that it is often very difficult to distinguish certain fossil species when the interior cannot be examined.
page 226 note * Identical specimens are found to Pike and Adams county, Illinois, America.
page 231 note * According to Mr., S.P. Woodwand, Lingnla would possess a pedicle miscle; there addnctor muscles, the posterior pair combined; two pairs of retractors, the posterior pair unsymmetrical, one of them dividing; and two poeterior sliding muscles.
page 233 note * I possess boo an American Lingual from the Pottsdana saridstone of the Falls of St.Minnesota, which is stated by Mr. Worthen to be the oldest kauwn American specles of the genus. In shape it is very similar to L. Scotica, but differs from it in sculpture as well as in the convexity of its valves.
page 223 note † This shell has been known for many years in Scotland as a Posidonomya, but of which genus it does not poesess the character.
page 234 note * At page 368 of his History of British Animals,” while describing Lingula mytiloides, Dr. Fleming refers to Ure's plate xvi., fig. 5, as probably belonging to the species in question, but I am convinced that this is a mistake, as the shell there represented is far from reasmbling any Iingola with which I am acqnainted.
page 237 note * Conditions arising from food and climate no donbt modify form and size, and modifications of form not amounting to malformation might arise from diseased condition.
page 240 note * Transactions of Berwickshire Club, vol. iii, p. 172.
