BIVALVIA. 55 



placed at right angles to the surface, while the inner is laminated; and as these laminae are 

 placed in an irregular, or rather in an undulating or wavy manner, they produce the 

 shelly substance called nacre, which, it appears, is more susceptible of destruction than the 

 outer or fibrous portion, and this in many of the secondary fossils is the only part of the 

 shell which is preserved. The fibrous portion being very fragile, the shell separates 

 readily into fragments ; and it is, therefore, a fossil not generally found in a perfect 

 condition. There is also another peculiarity in the shell, being, as it were, divided by a 

 line down the middle, from the pedilateral to the siphonilateral margins, where it will 

 readily crack : so that fossils have frequently become of a quadrate form ; such, for example, 

 as Pinna tetragona, which has assumed that shape from pressure alone. Many of the 

 species of this genus will show, more or less, this tendency to angularity on the outside of 

 the valves. The pearly or nacrous lining seldom extends more than half-way from the 

 beak ; frequently not so far. Many of the species attain to large proportions. Pinna 

 nobilis, of Poli, an inhabitant of the Mediterranean, has a shell that is said to measure three 

 feet and a half. This genus is as old as the Carboniferous Limestone, or perhaps the 

 Devonian; so far, of course, as can be determined by the shell alone; and it existed through 

 the intermediate periods, though not anywhere in great abundance. One of the aberrant 

 forms found in the Oolites has been thought, by Messrs. Morris and Lycett, to have a 

 difference sufficient to justify a generic separation, and they have adopted for it the 

 name of Trichites ; the history of which, and the reasons for adoption, are given by these 

 authors in their valuable monograph of the Oolitic Bivalves. The principal difference existing 

 between the Oolitic shell and the true Pinna is in the inequality of the valves. 



Pinna is on the verge of the ordinal division ; it more strictly belongs to the Dimyaria, 

 having two adductor muscles, but one is so near to the pointed extremity of the valves as 

 to be used more as a connector than as an adductor, while the other, the really useful one, 

 is situated quite in the centre of the shell. 



1. Pinna affinis, /. Sowerby. Tab. X, fig. 1, a— c. 



Pinna affinis. /. Sow. Min. Conch., t. 313, fig. 2, 1821. 



— — Mantel/. Geol. Tr., 2d series, vol. iii, pt. i, p. 203, 1829. 



— — Wetherell. Phil. Mag. and Journ., vol. ix, p. 464. 

 — Nyst. Foss. Belg., p. 275, 1843. 



— — J. Soiv. in Dixon's Geol. Sussex, pp. 31, 117, 172, 226, 1850. 



— — Morris. Cat. Brit. Foss., p. 179, 1854. 



— — D'Orbiyny. Prod, de Pale'ont., t. ii, p. 391, 1850. 



Spec. Char. P. testa cuneatd, trigone!, regulari, costatd ; apice acuto ; siphoni-regione 

 truncatd ; costis divergentibvs ; margine dorsali recto, margine ventrali sub-arcuato, 

 simplici. 



Shell cuneiform or wedge-shaped, regularly trigonal ; apex acute, truncated at the 



