246 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 
Schlotheim, from the Oolite of England, Russia, South America,’ and 
other countries. 
The original diagnosis refers to the shell as being subequivalve, 
subequilateral, and radiately costated, with a cardinal margin auricled 
at both ends; the cardinal area includes a triangular, central, oblique, 
and oval fossula; on the anterior side is a prominent byssal orifice ; 
the umbones are straight and acute. 
Eichwald appreciated the fact that this genus combined the 
characters of Ostrea, Pecten, Lima, and Spondylus. He states that it 
is lamellose as in Ostrea and Lima, and slightly irregular as in both 
these genera, but the ribs are more regular than in the Oysters. Both 
valves are moderately convex and nearly equal as in Lima, both have 
wide expansions as in Pecten, and the ribs are mostly furnished with 
tubular prolongations as in Spondylus. The large opening for the 
passage of the byssus distinguishes it from Zima and Spondylus, 
whilst the shell is not fixed by its apex as in the Oysters, but by 
the byssus as in Pecten. 
The geological distribution of Ctenostreon extends from the Lias 
through the Oolite period, when it attained its maximum development, 
into Cretaceous times; its occurrence in the Lias of Chili, South 
America, having been reported by Moricke? in 1894, whilst Stoliezka® 
has identified certain forms in the Cretaceous strata of Europe and 
India. 
CrenostREoN BurcKHARDTI, n.sp. (Plate V.) 
Shell large, thick, suborbicular, nearly equilateral, moderately 
convex, covered with undulating lamelle of growth, coste few 
(7 or 8), radial, elevated, rounded, forming fistulous plications and 
terminating in large cylindrical spines; interstitial spaces wide, 
excavated, equal; posterior ears vertically ridged, and with a well- 
defined border; anterior ears with a thickened, reflected margin, 
forming an elongate byssal orifice. 
Dimensions of opposite valves belonging to different individuals :— 
Calculated Calculated 
without spines. with spines. 
{ Umbono-ventral See 134mm. or 195mm. 
Lerr Vatve / Antero-posterior a 125 |, ae IGS, 
Depth, about... age 40 ,, 
Umbono-ventral es 15075, Se ae Bs 
Rieut VAuLveE ( Antero-posterior “oe LDR s ae 70s 
Depth, about ... Ape 45 
The principal points of difference between this shell and C. pectini- 
formis, to which it is most nearly related, are its fewer coste, the 
presence of wider interstitial spaces, and the stronger development of 
the spinous prolongations. For purposes of comparison the number 
of ribs on the chief Jurassic species may be referred to as follows :— 
1 G. Steinmann: Neues Jahrbuch, 1881, Beilage Bd. i, p. 256 (Bolivia). 
2 ««Versteinerungen des Lias und Unteroolith von Chile ’’: Neues Jahrbuch, 1894, 
Beilage Bd. ix, p. 35. 
3 «« Cretaceous Pelecypoda of Southern India’’: Paleontologia Indica, 1871, 
pp- 414-416, 422. 
