Hyatt.] 
344 
[Dec. 2. 
curve toward the venter. The nacreous layer reaches to the 
hinge line dorsally but falls short of the ventral border leavjhg 
a zone of the fibrous layer visible, Avhich is continuous, with the 
exposed fibrous layer of the posterior region beyoud the nacrejms 
layer. The nacreous layer masking the carinae internally nfear 
the apex, is built across them smoothly in the young or with a 
slight internal keel, but in full grown specimens with thicker 
nacre there are often short prominent keels formed internally, 
which terminate at the coecal end of the median split. 
The two regions on either side of the carina in each valve 
have been named the ventral area and dorsal area. 
I have been unable so far to ascertain positively that species 
of this genus existed in the Carboniferous. 
P. miliaria and a species allied to Inartmanni sp. Stoppani, 
are found in the Trias. 
P. hartmanni Zieten, maxima Eichwald, occur in the Jura. 
P. abrupta and subcuneata Eichwald, restitua Hoenig, de- 
pressa Goldf., complanata and intumescens Stol. arata Forbes? 
breweri Gabb, are from the Cretaceous. 
P. affinis Goldf. (not Sowerby) and multisulcatus Mayer- 
Eymer are from the Tertiary. 
Among existing species there are two groups. The typical 
Pinnas have thick shells, coarse longitudinal folds, and often 
very large, prominent, and usually not very thickly set scales. 
These are as follows, P. rudis Linn., pernula Chetnn., jlabellum 
Lam., rug osa Sow., fimbriatula Reeve, 1 and electrina and zebu - 
ensis Reeve. 
The second group has fine longitudinal ridges with small and 
often closely crowded scales, and the outlines are somewhat differ- 
ent as a rule from those of the first subdivision. The apex is not 
so narrow in proportion to the body of the shell, or in other words 
the body of the shell does not expand quite so rapidly especially 
on the ventral border. 
Pinna murieata is stated to be the type of Pennaria, Browne, 
1756, but we have not the means of verifying this reference. 
The name was used by Morch in 1853. 2 
1 The scales, large ribs, and form of fimbriatula are similar to those of rudis but the 
figure does not enable us to say positively that it has carinae. 
2 Op. cit., p. 51, Morch refers murieata pectinata Linn., nobilis, inflata, andsac- 
cata Chemn., seminuda Lam., rigida Dill wyn, to this genus. 
