244 



GENERIC DISTRIBUTION. 



and south-west shores with T. umbilicatus and 

 lineatus. 



To this zone, when rocky, belong Patella vulgata, 

 Skenia planorbis, and Kellia rubra. In brackish 

 water Rissoa ulva swarms. 



The second is the " Laminarian zone," from the 

 abundance of that and other sea- weeds, extending 

 from low water to 15 fathoms. The genera Lacuna 

 (one species excepted), Calyptrasa, Aplysia, Scrobi- 

 cidaria, and Donax, do not range in our seas below 

 this belt. Rissoa, Chiton, Bulla, Trochus, Mactra, 

 Venus, and Cardium, are at their maximum here. 



The third is the " Coralline zone," reaching from 

 15 to 50 fathoms. Vegetation is scarce, and ulti- 

 mately disappears within these limits ; and the zone 

 takes its name from the hydroid zoophytes. Tro- 

 chus ziziphinus and T. tumidus, Chiton asellus, Acmcea 

 virginea, Turriteila communis, Venus ovata and 

 V. fasciata, Pecten opercularis, Pectuncidus glyci- 

 meris, and Nucula nucleus mark the upper por- 

 tion of this zone. Solen pellucidus, Pecten varius, 

 Modiola modiolus, Dentalium, and Mactra elliptica 

 occur lower. 



Genera of Testacea have also their characteristic 

 zones of depth. As we draw nearer to the present, 

 in following out the sequence of fossil forms, a 

 system of representation in time becomes distinctly 

 marked, so that it would not be difficult to arrange 

 the species of many tertiary groups of strata bathy- 

 metrically, according to the known conditions of 



