252 REPORT—1850. 
as solitary individuals in situations where the conditions do not seem so 
favourable to fecundity. Many species also, not at all gregarious in the true 
sense of the word, having a very wide range in depth, are not equally pro- 
lific throughout that range, but are developed in much greater numbers in 
one region than in another, or in different parts of the same region accord- 
ing to the conditions of the sea-bed. Climatal differences also have a con- 
siderable effect in determining the prolific or non-prolific character of a 
species, and this may be observed clearly, even in such a limited area as that 
under review. Hence, when we state of many species that they-are diffused 
throughout all the provinces of that area, it is not to be understood that they 
are equally abundant, so far as their individuals are concerned in all. Thus, 
for example, Dentalium entalis is distributed throughout the British seas ; but, 
whilst it is so abundant as to be almost gregarious in the northern provinces, 
it becomes scarce and solitary in the southern. Many examples of this may 
be seen by consulting the analysis of dredging papers in the preceding tables, 
and afterwards comparing them with the tables of enumeration of localities 
of species. 
In the Littoral region, as mentioned already, the species of Littorina, Tro- 
chus, Patella and Purpura are most abundant, and among bivalves, Mytilus 
edulis, Cardium edule and Kellia rubra. ‘These, with many other animals, 
and with peculiar marine plants, which it is not the province of this Report 
to enumerate, give a character to the sea-belt between tide-mark*. 
In the Laminarian region, extending from low-water mark to 15 fathoms or 
thereabouts, Lacune and Rissoe are abundant. The species observed to be 
most prolific within this region during the dredging researches on the English 
shores, were Rissoa parva and interrupta; in Laminarian shallows, Lacuna 
puteolus, Rissoa labiosa and Phasianella pullus, where Zostera prevailed ; 
Trochus cinereus, Magus and Ziziphinus, Acmea virginea, Modiola modiolus, 
Nucula nucleus on muddy gravelly bottoms; Turritella, Corbula nucleus, 
Syndosmya alba, Dentalium tarentinum, Ophiocoma rosula in sandy and 
muddy places; Solen pellucidus and Mactra subtruncata where sand pre- 
vailed; Chiton asellus everywhere where shells or stones were present ; 
Echinus miliaris on many bottoms; Ascidie and Crustacea everywhere. 
On the Scottish shores in like depths, most of the above-named forms 
(except the Rissoa labiosa, Phasianella, Lacuna and Dentalium tarentinum) 
were equally prolific, whilst others seldom observed in great numbers in the 
south became very plentiful, as Dentalium entalis, Lucina flexuosa, Lima 
hians, Venus striatula, Ophiocoma chiagit ; and in places, Cardium pygmeum, 
Crenella decussata and Bulla akera. 
Between 15 and 25 fathoms in the upper part of the Coralline zone, 770- 
chus ziziphinus and tumidus, Chiton asellus, Aemea virginea, Nassa reticu- 
lata, Turritella, Venus ovata and V. fasciata, Pecten opercularis, Modiola 
modiolus, Crenelle, Pectunculus, Nucula nucleus, abound in individuals on 
the English shores. The same species, with the addition of Astarte sulcata 
and A. elliptica, Syndosmya intermedia, Lima subauriculata, Leda caudata, 
Cardium fasciatum and Lucina sinuata, mark the same region in the Scottish 
seas. Jn both north and south Echinus sphera and Ophiocoma are very pro- 
lific in this belt. 
Between 25 and 40 fathoms, in the middle and lower sections of the 
Coralline region, the species observed most prolific in individuals on the 
English coast were few, comprehending Solen pellucidus, Pecten varius, 
Modiola modiolus and Dentalium tarentinum. 
* For a tabulated view of the subdivisions and inhabitants of this zone, see the first. 
volume of the Memoirs of the Geological Survey of Great Britain. 
