Hyatt.] 
342 
[Dec. 2. 
Kildare, Ireland. Coll. Mus. Comp. Zool. Cambridge, Mass. 
The distribution of the nacreous layer could not be studied 
satisfactorily. In one specimen the posterior border of the pos- 
terior muscle was apparent and if this indicated the posterior 
border of this layer it must have been quite short occupying 
the apex but not longer than in broader forms like S. hartmanni 
of the Jura. It is very evident that this layer was neither so 
thick nor so important as in the shells of later times. 
Besides this species there are also in the Carboniferous some 
elongated shells without carinae, such as S. sancti-ludovici sp. 
Worthen, missouriensis sp. Swall., inexpectans sp. Walcott, 
ludlovi and maxvilliensis sp. Whitf., and jlabelliformis sp. De 
Kon. 1 (pars). Some of these have the convex zone so faintly 
marked that it is difficult to see it and in others it seems to be 
absent, but on the other hand the Carboniferous forms are very 
distinct from the Triassic and Jurassic species in their elongated, 
almost cylindrical, heavily ridged shells with smooth hinge 
cresses, and it seems impracticable to divide them between Suica- 
topinna and Atrina. 
I have not been able to find any species in the Trias or Jura 
which can properly be referred to this genus but if one can trust 
figures alone, Pinna gallieni and quadrangular is sp. D’Orb. (not 
Goldfuss) of the Cretaceous may possibly belong to this genus. 
Pinna. There is a gap between Sulcatopinna and the carinated 
Pinnas which it is difficult to fill. 
A cast of the protean form usually named Sulcatopinna mis- 
souriensis occurs in the Coll, of the Mus. of Comp. Zoology, 
which has been compressed dorso-ventrally and has on each side 
a decided but narrow carina made by a line of fibrous matter 
which is buried in the surface of the cast, and one can see that a 
long double fold is the external expression of this structure. 
Double median folds occur in another specimen referred to Sul- 
catopinna from Chester, Illinois, on one side of the cast and a 
raised double ridge on the other. Both of these are characters 
acquired late in the life of the shell near the end of the nacreous 
layer, the muscular impressions being visible on this cast. The 
1 Pinna costata , sp. Phill., inaequicostata , Me Coy, and jlexicostata' also of Me Coy 
are said by De Koninck to be identical with varieties oi S. jlabelliformis, but this seems 
to me to be giving almost generic significance to the specific characters of S.flabelli- 
f or mis. 
