17() 



CONCHIFERA. 



Unio. 



Transversely oblong-ovate ; anterior side short, hardly ex- 

 tending beyond the beaks, which are obtuse ; posterior side 

 obliquely rounded, and angulaied below ; surface with fine, 

 numerous, radiating striie, crossed by sharp, pretty regular 

 lines of growth, proilucing a reticulated appearance. 



In the Bath Oolite, in various localities : it is very frequently 

 found in numbers of four or five individuals, enveloping one 

 another, within the Modiola fwc/«s«, in the manner represented 

 in fiff. 23. 



Order II.— DIMYAEIA. 



Shells with two distinct, remote, muscular impressions ; 

 which are widely separated, and inserted towards the lateral 

 extremities of the valves. 



Grand Division I. 



Shells irregular, and always inequivalve. 



Tribe I.— CHAMACEA. 



Shell inequivalve, irregular, attached to other bodies ; hinge 

 with one or more large teeth, and provided with two separate 

 lateral muscular impressions. 



Genus I. — CHAMA. — BrvguUre. 



Shell irregular, thick, usually very inequivalve, for the most 

 part covered with irregular spines or foliated processes ; um- 

 bones distorted, unequal, distant, and involute ; that of the 

 attached valve salient at the base, and in some instances pro- 

 jecting considerably beyond it, the other is for the most part 

 reflected over upon its valve, appearing as if imbedded in it ; 

 hinge with one strong, thick, irregular, oblique, striated, and 

 generally crenated tooth in one valve, which fits into an irre- 

 gular striated groove in the opposite valve ; each valve pro- 

 vided with two distant, lateral, muscular impressions ; line of 

 the mantle attachment entire ; ligament external, subdivided 

 at its posterior extremity ; one of the segments decurrent to 

 the point of the umbo in each valve. 



1. Cha.ma squamosa. — The Scaly Chama, pi. LXXII."' 

 figs. 4, 5. 



C. squamosa. Brander, figs. 86, 87. Sowerby, IV. p. 67, 

 pi. 348. 



Sub-globose, or nearly orbicular, attached by the right valve, 

 which is somewhat larger than tho left ; surface with nume- 

 rous transverse, imbricated, erect lamina, anteriorly produced 

 and adpressed ; posterior portion of the right valve with obso- 

 lete ribs ; left valve rather convex ; inner surface smooth. 



The London Clay, Barton and Bracklesham Bay. 



[Genus II.— CAPRINA.— Z)'Orii>«y. 

 Shell irregular, inequivalve, inequilateral, with conical 

 divergent apices, more or less unequally prolonged, and in- 

 curved upon two opposite planes ; hinge and liizament un- 

 known ; cavity of the valves divided by a partition into two 

 conical unequal chambers ; two muscular impressions situated 



within the small cavities, the one before and below, and the 

 other above and behind. 



] . Caprina Lonsdalii. — Lonsdal's Caprina, pi. LXXII.* 

 figs. 10, 11. 



Diceras Lonsdalii. Sowerby, Geo. Tr. 2d Ser. IV. p. 33S, 

 pi. 13, fig. 4. 



Inequivalve, the larger one in the form of an elongated cone, 

 somewhat flattened, and curved twice round ; the opposite 

 valve with an oblique conical umbo ; external surface squa- 

 mose. 



The Lower Greensand, near Calne. 



Grand Division II.— LAMELLIPEDES. 



The foot of the animal depressed, lamelliform, and not 

 posterior. ■ 



Tribe I.— NAYADES. 



Shells inhabiting fresh waters. — Hinge sometimes provided 

 with an irregular, simple, or divided tooth, and a longitudinal 

 prolonged one ; sometimes toothless ; some have irregular 

 granulated tubercles, extending the whole length of the hinge- 

 line ; provided with a compound muscular impression ; the 

 umbones or beaks frequently decorticated. 



Genus III. — ANODON. — Bruguiere. 



Shell equivalve, inequilateral, and transverse, for the most 

 part very thin ; hinge-line nearly straight ; destitute of 

 cardinal teeth ; the hinge being glabrous, and provided with 

 smooth laminae ; truncated, or forming a sinus at the anterior 

 end, terminating the apex of the shell ; two lateral, remote, 

 muscular impressions, the posterior one being compound ; 

 muscular impression of the mouth entire, and seldom dis- 

 tinctly marked ; ligament linear, external, sunk in a cleft at 

 the anterior extremity. 



1. Anodon cygnea. — The Swan Anodon, pi. LXXIV.* 

 fig. 7. 



Anodon cygnea. Brown, Land and Fresh-water Conch. 

 Brit. p. 101, pi. 13. 



Inequilateral, oval, tumid, somewhat pointed at both ex- 

 tremities, slightly open at the sides ; beaks depressed ; surface 

 transversely wrinkled and sub- striated. 



In the Pleistocene Fresh-water Formation, Cropthorn; Bac- 

 ton, Stutton ; Clacton and Grays. 



Genus IY. — UNIO. — Bruguiere. 



Shell generally transverse, equivalve, inequilateral, free ; 

 sometimes sub-cordate, or sub-orbicular; pearlaceous within; 

 generally covered with a dark olivaceous epidermis ; umbones 

 usually decorticated and prominent ; hinge provided with a 

 short, irregular, simple, or double compound tooth, which is 

 almost always striated ; with two elongated, compressed, late- 

 ral teeth, the front one produced, sometimes obsolete ; two 

 muscular impressions in each valve, the superior one compound, 

 or composed of several divisions ; ligament external. 



