BIVALVIA. 55 
placed at right angles to the surface, while the inner is laminated ; and as these laminz 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. he 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. Pina 
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 Zrichites ; 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 arrinis, J. Sowerby. Tab. X, fig. 1, a—c. 
Pinna aFrinis. Jd. Sow. Min. Conch., t. 313, fig. 2, 1821. 
— — Mantell. Geol. Tr., 2d series, vol. ili, pt.i, p. 203, 1829. 
— — Wetherell. Phil. Mag. and Journ., vol. ix, p. 464. 
- — Nyst. Foss. Belg., p. 275, 1843. 
— — J. Sow. in Dixon’s Geol. Sussex, pp. 31, 117, 172, 226, 1850. 
— _— Morris. Cat. Brit. Foss., p. 179, 1854. 
= — D' Orbigny. Prod. de Paléont., t. ii, p. 391, 1850. 
Spec. Char. P. testa cuneatd, trigond, regulari, costata ; apice acuto ; siphoni-regione 
truncata ; costis divergentibus; margine dorsali recto, margine ventrali sub-arcuato, 
simplict. 
Shell cuneiform or wedge-shaped, regularly trigonal; apex acute, truncated at the 
