20 BRITISH CARBONIFEROUS BRACHIOPODA. 



and depth. The hinge-line is either a little shorter, or as long as the greatest width 

 of the shell, the cardinal angles being more or less rounded in adult individuals. 

 The area is of moderate width, with sub-parallel sides ; fissure triangular, and partially 

 covered by a pseudo-deltidium. The external surface of the shell is ornamented by a 

 variable number of radiating ribs, which augment in number, to a greater or lesser 

 extent, from intercalations at unequal distances from the beaks ; so that from seventy to 

 ninety may be counted round the margin of each valve in adult individuals. The 

 ribs on the fold and sinus are likewise more flattened than on the lateral portions of 

 the shell. The surface is closely and finely reticulated. In the interior of the dorsal 

 valve, under the extremity of the incurved umbonal beak, there exists a small cardinal 

 process or muscular fulcrum, and on either side are situated the dental sockets. The 

 spiral cones which fill the larger portion of the shell are attached to the extremities of 

 the inner socket- walls. The lamellae, after having converged and given birth to the 

 crural processes, diverge, and form the first of the twenty or twenty-two convolutions of 

 which each spiral is composed. Four impressions left by the adductor muscle are visible 

 in this valve. In the interior of the ventral valve a strong hinge-tooth is situated on either 

 side at the base of the fissure, and is supported by a vertical shelly plate of much strength, 

 but not advancing to any great length into the interior of the valve. Between these a 

 large portion of the free space at the bottom of the shell is occupied by the adductor and 

 cardinal muscular impressions, which are divided by a blunt, central, longitudinal ridge. 

 The dimensions of one of the largest examples are — 



Length 4^ inches, width 6 inches 1 line, depth 3 inches 1 line. 

 05s. In the opinion of M. De Koninck, De Verneuil, M'Coy (1855), as well as of 

 other authors, Sp. attenuata, Sow., must be considered as a synonym of Sp. striata of 

 Martin ; and in this view I am the more disposed to concur, from having been able to 

 assemble and study upwards of four hundred individuals of this shell, from the dimensions 

 of a few lines to those of the largest example above recorded. With so numerous 

 a series before me, all minor differences, so remarkable when particular or exceptional 

 forms are placed in comparison, soon vanished, and I can see in them nothing further 

 than those dissimilarities so common to individuals of every species composing the 

 animal kingdom ; for it is a well-known fact that no two examples are ever found to be 

 so exactly similar as to induce one to suppose that they were cast in the same mould, 

 and it has also been well ascertained that under certain conditions individuals of most 

 species may become adult with much smaller dimensions in one locality than in another. 

 The number of ribs or external sculpture varies also to a considerable extent in different 

 specimens. This is no doubt the reason why at Millecent (Kildare) we may meet with 

 upwards of a thousand middle-sized specimens for one large individual, while in other 

 localities the proportions are on an average much larger. The name Sp. princeps 

 was given by Professor M'Coy to a full-grown example of Martin's striata, and must 

 therefore be added to the synonyms of that form. M. De Verneuil is also of opinion 



