MOLLUSCA. LAMELLIBRANCHIA 229 



by a groove. Posterior to the binge is an internal, elongated 'ossicle.' 

 External ligament small. Pallial line simple. Devonian and 

 Carboniferous. Ex. E. unioniformis, Carboniferous Limestone. 



Sanguinolites. Shell elongate, very inequilateral, with 

 rounded ends, the posterior part usually higher than the anterior 

 part ; surface with concentric ribs or lines. Umbones near the 

 anterior end, with a ridge passing to the lower part of the posterior 

 end ; lunule and escutcheon distinct. Anterior adductor impression 

 large, deep, limited posteriorly by a ridge ; posterior adductor 

 shallow, near the hinge. Pallial line entire. Hinge toothless. 

 Carboniferous. Ex. >S'. angustatus, Carboniferous Limestone. 



Distribution of the Lamellibranchia 



All the Lamellibranchs are aquatic animals, and by far 

 the larger number are marine. The marine forms range 

 from the shore-line down to a depth of 2900 fathoms ; 

 they are most abundant in shallow water, and are scarce 

 at depths greater than 500 fathoms, but the following, 

 and a few other genera, have been found below 1500 

 fathoms: — Nucula, Nuculana, Area, Limopsis, Malletia, 

 Verticordia, Guspiclaria (= Necera). 



Two genera of Lamellibranchs have been recorded 

 from the Lower Cambrian of North America ; in England 

 the earliest forms appear in the Tremadoc Beds. They 

 are rather rare in the Ordovician, but become fairly 

 numerous in the Silurian, and afterwards gradually in- 

 crease in importance, reaching their maximum at the 

 present day. Many of the genera have a rather extended 

 range in time. 



In the Palaeozoic formations the Taxodont and 

 Dysodont groups, and primitive dimyarian forms with 

 imperfectly developed hinges, are important. In the 

 Carboniferous period Carbonicola and its allies and the 

 Pectinida? are well represented ; only a very few forms with 



