i)S Professor Edward Forbes on 



lieport on the Investigation of British Marine Zoology by 

 means of the Dredge. Part I. The Infra-littoral Distribu- 

 tion of Marine Invertebrata on the Southern, Western, and 

 Northern Coasts of Great Britain. By Professor Edward 

 Forbes. 



(^Continued from page 391 0/ Vol. LI. of Philosophical Journal) 



Gregarious and fvolific species. — Many of our littoral mollusca, 

 as the shore-living species of £ii^orma, Purpura, Trochus, Cardium, 

 DonoM, Mya, Pholas, &c., are truly gregarious, and the individuals of 

 each are constantly found assembled together in considerable numbers. 

 This . is not so commonly the habit among sub-littoral species ; 

 among them, however, there are some habitually gregarious (as 

 Ostred edulis, &c., and among radiata, Ophiura rosula, Uraster 

 rubens, Comatula europcea, Echenus sphaera), though with this 

 difference as compared with the most littoral gregarious forms, that 

 whereas the individuals of the latter are always assembled together, 

 the sub-littoral species are gregarious in some zones of depth and under 

 certain conditions of sea -bottom, while they are at the same time 

 diffused in small numbers, or even as solitary individuals in situa- 

 tions 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 prolific 

 throughout that range, but are developed in much greater numbers 

 in one region than in another, or in different parts of the same region, 

 according to the conditions of the sea-bed. Climatal differences also 

 have a considerable 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, 

 Trochus, Patella, and Purpura are most abundant, and among 

 bivalves, Mytilus edulisy Cardium edide, and Kellia rubra. These 

 with many other animals, and with peculiar marine plants, give a 

 character to the sea-belt between the tide-mark. 



