Hyatt.] 
340 
[Dec. 2, 
Leaving out all but essential characteristics it can be readily seen 
that the nacreous layer is entire in all the shells of this genus, and 
that the posterior muscular impressions are very broad and accom- 
panied by single, small auxiliary impressions on the branchial sides. 
It is obvious that the essential characters of Atrina in the 
adult stage are probably repeated in the transient nepionic period 
of the carinated Pinnas before the carinae appear. The word 
“probably” is used because no one has, as far as I know, yet 
seen or figured a carinated Pinna in this stage, although there are 
positive indications in several species that such a primitive acari- 
nated condition was present at the apex. 
The genus is represented by A. hinrichsiana , a broad larval 
form with few ridges, and A. inflata sp. Phil., which may be 
named A. phillipsi , in the Carboniferous. 
A. prisca sp. Goldf. occurs in the Trias. 
A. trigonata Mart., cancellata Morris and Lycett, crumenila 
sp. Dumort., granulata and ampla sp. Sow., are found in the 
Jura. 
A. lakesi sp. White, laticostatus Stoliczka, rostiformis sp. 
Mart., ligeriensis , sulcifera , morcana and neptuni sp. D’Orb., are in 
the Cretaceous. 
A. margaritacea sp. Lam., pectinata sp. Woods, affinis sp. Sow., 
brocchi sp. D’Orb., arcuata sp. Sow., and transversa D’Arch., 
are in the Tertiary. 
Among existing species are the following: — 
A. nigra sp. Chemn., gouldi sp. Han., vexillum sp. Born., in- 
Jlata and papyracea sp. Chemn., hanleyi , lurida , and subviridis 
sp. Reeve, serrata , lanceolata , and alta sp. Sow., seminuda sp. 
Lam., carolinensis , assimilis , and strangei sp. Han., pectinata sp. 
Linn., zelanica sp. Gray, rigida sp. Dillwyn, deltodes sp. Menke, 
tuberculosa Sow. and Reeve. A rigida and alta have heavy ridges 
covered by scales while varieties of nigra and ingens have 
smooth exteriors. A. seminuda has finely set small scales. 
Streptopinna was described by Ed. von Martens 1 who violated 
the ordinary customs of conchologists in so far as he not only 
mentioned the type, P. saccata Linn, and Reeve, but also gave 
the characteristics of the genus, mentioning that it had an 
irregular form and that carinae were absent. The collectors of 
1 Meers f. d. Ins. Maurit. u. d. Seychellen, p. 318, 1880. 
