337 
[Hyatt. 
1891.J 
that on the dorsal area but the characteristics to be derived from 
the study of the outlines of the nacre on these two areas can cer- 
tainly be used to distinguish species in some cases. 
The internal posterior area occupied by the naked fibrous layer is 
continuous with a zone on the ventral border reaching to the apex, 
which is also unsupported by nacreous deposits. In most full- 
grown shells the fibrous layer is absent from the exterior near the 
apex, having been worn off by attrition. 
It is evident that the nepionic stage in all carinated forms is 
without carinae, since although shells at these very young stages 
have not been studied, the carinae are less developed at the apex 
and must have arisen during the later nepionic or early nealogic 
stages. 
It is also equally evident that the young were smooth, since 
the gradual introduction of longitudinal ridges beginning always 
near the dorsal border may be studied in most species. Jackson 
lias shown that the young have an extended hinge line indicating 
the existence of a transient stage similar to Palaeopinna of the 
Devonian. 
The appearance of carinae in the shells of this family during 
the Trias and early Jura shows that Pinna was even then distinct 
from the acarinate forms. Unfortunately the apex in all speci- 
mens of the Pinnidae is destroyed by attrition. This is doubt- 
less due to the habit of living partly buried in the sand. There 
is therefore no way of proving that the umbones are absolutely 
terminal as has been generally asserted in all descriptions. The 
umbones are doubtless more nearly terminal than in Aviculopinna, 
but it is not safe to go beyond this assertion and even that is an 
inference from the general outline of the shell and the lines of 
growth in young shells and has not been proven by direct obser- 
vation . 
The hinge occupies the dorsal side and is internal, consisting 
of the usual elastic cushion of fibrous conchiolin coextensive with 
and inside of the nacreous layer. Adults of species of Atrina, 
having, like A. nigra , comparatively short straight hinge lines, 
may have the power of opening the shell to some extent in adults 
and old specimens, but the irregularity of this line or its concav- 
ity in many species shows that it is not used. The mechanism 
of opening the shell by the elasticity of an internal cushion of 
conchiolin cannot operate with an irregular hinge line or a con- 
22 APRIL, 1892 . 
PROCEEDINGS B. S. N. H. 
VOL. XXV, 
