i8gi.] 
345 
[Hyatt. 
Other species of this group are P. d’orbigni , hanleyi , 
angustana Lam., nobilis , rotundata , aculeata Chemn., squamosa 
Gmelin. 1 P. seynicostata Reeve has a truncated or triangular 
outline when seen from the side, a peculiarity observable in 
other species. If P. saccata Chemn., and aequilatera von 
Martens, are distinct there are at least three species having this 
truncated form. 
Cyrtopinna. — This group was established by Morch, 2 in 
1853, the type being the well-known Pinna incurvata Chem- 
nitz. The shells are very long and narrow and sometimes have 
extremely long carinae, the nacreous layer being in such cases 
also very long in proportion to the whole shell. 3 The nacre- 
ous layer in some species, incurvata , menJcei , vespertina , mutica , 
rumphii , extends a considerable distance beyond the muscular 
impressions on the dorsal area and although this sometimes oc- 
curs in Atrina and to a less extent in some species of Pinna it 
is evidently more general in Cyrtopinna than in other genera. 
There are very faint longitudinal ridges on the dorsal area and 
none or very few on the ventral area close to the carina in some 
species. The aspect of the shell in these forms is smooth with 
the exception of the transverse wrinkles which appear more or 
less at all stages as in all the Pinnidae and are especially promi- 
nent in the old age. On the other hand the shells may have 
longitudinal ridges and even be scaled, but there is a marked 
absence of scales even in geratologous stages, and in most of the 
species at earlier periods of growth they are invariably absent. 
These characteristics are in strong contrast with those of 
Pinna and it is practicable to separate the species of this group 
from those of that genus. There are two groups of this genus. 
The species of the first subdivision have very elongated narrow 
valves, not broadening suddenly at the posterior part although 
1 It is not usual to distinguish so many of these forms but in the collection of the 
Boston Society of Natural History there are shells exhibiting marked differences which 
appear to represent the species as given and I have provisionally separated them. 
The nacreous layer is often distinct in its outline and distribution when the exterior of 
the shell is almost exactly identical with that of another species. 
2 Note I. Cat. Conch, quae rel. D’Alphon. d’Aguirra et Gad. Comes de Yoldi, Pt. 
II, Aceph. p. 51, 1853. He mentions only C. incurvata Chemn. and C. incurva Gm. 
3 These nacreous layers in some species of Pinna and Atrina ipay be of equal 
length compared with the whole shell. 
