80 The Irish Naturalist. [April, 



The range of depth of some, species is extraordinary, that 

 of Tercbratula vitrca (Testicardinate), which has an almost 

 cosmopolitan distribution, being from 5 to 1,450 fathoms. 

 This great vertical range may perhaps be accounted for on 

 the assumption that it was brought about gradually ; in some 

 cases perhaps, in slowly sinking areas of the ocean bed, 

 so that the organism had time to accommodate itself to an 

 abyssmal habitat. The Brachiopods are abundant to a depth 

 of about 500 fathoms, beyond this they become scarce, but it 

 must be borne in mind that the amount of the sea-bed explored 

 by the dredge is infinitesimal compared with its whole area. 



Among Carboniferous Brachiopods the following are im- 

 portant and characteristic : — Productus, Spirifcr, Athyris, 

 Terebratula, Rhynchonclla. First of these in the number and 

 variety of its species is Productus, which is an invariable 

 accompaniment of the Carboniferous Limestone wherever this 

 rock may be found. So abundant is it in some beds as to 

 give its name to them ; the " Productus Limestone " of the 

 Salt Range of the Punjab is a case in point, though it should 

 be stated that this particular deposit forms part of a system of 

 rocks — the Permian — which was laid down subsequently to the 

 Carboniferous, containing, however, fossils (Productus amongst 

 them), which are clearly the descendants of Carboniferous 

 species. 



The great local accumulation of one kind of shell in the 

 case referred to, and it is not an isolated one, indicates a 

 condition of things not dissimilar from that which is met 

 with at the present day in the vast oyster and mussel beds of 

 our shores. 



In the species Productus giganteus we have the largest 

 known Brachiopod, some individuals measuring as much as 

 eight inches across. Productus scmircticulatus is the com- 

 monest species, and is easily picked out from among its 

 fellows by means of the beautiful system of thread-like lines 

 covering the surface of the shell and forming a net-work on 

 the upper half (hence the name), where the lines radiating 

 from the beak are crossed by those added to the margin of the 

 shell as it grew, called the " lines of growth." The larger 

 (ventral) valve is convex, the smaller (dorsal) valve concave, 

 leaving apparently very little space for the animal between 



