REPTILIAN AGE. JURASSIC PERIOD. 103 



tab. 1832 ; ib. 756 ; again in Lamk. 2d ed. VI, 63.— F. A. Roemeh, Verst. Ool. 1836, 126.— Beonn, Leth. 



1837, 384.— Agassiz, Etud. Crit. Mol. Foas. 2d liv. 1842, 37.— Hanley, HI. Cat. 1844, I, 18, &o. &o. 

 Pholadomyma, Fleming, Hist. Brit. An. 1828, 408 and 424. 

 ? Cymella, Meek, Smithsonian Chk. List Cret. Foss. 1864, 34. 

 Etym. — Pholas ; Mya. 

 Type. — P. Candida, Sowekby. 



Shell thin, equivalve, pearly within ; longitudinally oblong, oval, or subtrigonal; 

 inequilateral and ventricose; extremities usually both gaping, but the posterior 

 more widely than the other. Surface ornamented with radiating costae, crossed by 

 concentric strise, or more or less distinct, sometimes nodular, ribs. Hinge with an 

 obscure lamellar tooth, and a small triangular pit in each valve. Ligament short, 

 external. Pallial line and muscular impressions generally faintly marked; the 

 latter broadly sinuate. 



The animal of Pholadomya Candida, according to Owen, has its mantle provided 

 with four openings — a pedal, a siphonal, and an anal aperture, with a fourth small 

 circular orifice at the under part of the siphons. The gills are thick and finely 

 plaited, the outer lamina being extended dorsaUy. The foot is provided with a 

 small accessory bifurcating appendage behind. 



Prof Agassiz, who has produced the most important work on this genus, sepa- 

 rates the species into two principal sections, and again divides each of these into 

 several subordinate groups, as follows : — 



Section I. — Species Without a Circumscribed Cardinal Area. 



1. "Multicostatae," Agassiz. 



Shell more or less elongated ; radiating costae numerous, well defined. 

 Pxcnitples. — P. semicostata and P, multicostata, Ag. (^Jurassic and Cretaceous.) 



2. " Trigomatae," Agassiz. 



Shell subtrigonal or more or less oblong ; anterior side gibbous, often closed ; posterior more compressed and 

 distinctly gaping ; beaks generally elevated, sometimes perforate. Surface with distinct concentric ridges, 

 often extending upon the extremities of the valves, and crossed upon the flanks by well-defined tubercular 

 radiating costae ; cardinal area distinct, but not sharply defined. 



Examples. — P. arcuata and P. nuda, Ag. {Cretaceous, Tertiary, and Recent.} 



3. " Bucardinae," Agassiz. 



Shell subtrigonal, or more or less ovoid, gibbous, and comparatively thick ; truncated, somewhat gaping, 

 and usually flattened anteriorly ; posterior side distinctly gaping ; beaks gibbous and closely contiguous. 

 Surface with strong, often tubercular radiating costae on the flanks ; muscular and pallial impressions 

 distinct. 



Examples. — P. cincta and P. decussata, Agassiz. (_Lias to Tertiary.) 



Section II. — Species With a Circumscribed Cardinal Area. 



4. "Flaliellatae," Agassiz. 



Shell usually much elongated ; radiating costae prominent and often distant, generally confined to the flanks. 



General aspect similar to the " Multicostatm," but diflfering in the possession of a defined cardinal area. 

 Examples. — P. pelagica and P. similis, Agassiz. (^Jurassic.) 



6. " Ovales," Agassiz. 



Shell ovoid, more or less compressed ; one or the other of the extremities gaping : cardinal area sometimes 



not very distinct behind ; costae linear, often orenate. 

 Examples. — P. tenuicostata and P. pectinata, Agassiz. (^Jurassic.) 



6. " Cardissoides," Agassiz. 



Shell subtrigonal, similar to the " Bucardince" of the first principal division, but differing in the possession 



Of a defined cardinal area, and in having the costae less numerous and more faintly marked. 

 Examples. — P. cancellata and P. cardissoides, Agassiz. (^Jurassic.) 



