156 BRITISH CARBONIFEROUS BRACHIOPODA 



still be seen in Dr. Fleming's collection. P. spinosus is another condition or variation 

 of the shell under description, while P. lobatm represents those specimens in which the 

 median sulcus is very much developed, while every connecting link can be found in the 

 same bed and quarry between the evenly convex specimens and those with a deepened 

 median sinus. The striation is sometimes very fine in some examples, coarse in others, 

 so that on two shells of the same dimensions fifty ribs will be counted on the ventral 

 valve of the one, thirty on the other; the hinge-line is also in many examples shorter 

 than the width of the shell, longer in others. The long, slender, tubular spines which 

 are scattered over its surfaces vary likewise considerably in number in different individuals, 

 and Sowerby was almost correct while mentioning that " two principal ones are nearly 

 straight and cylindrical, attached to the convex side of the convex valve, and extending in 

 line parallel to the hinge." These two spines are rather constant in the majority of specimens 

 I have examined, and are often seen as represented in Sowerby's figure (5 of my plate), but 

 not always parallel to the hinge ; the other spines are more irregularly scattered over the 

 surface, while some are two or even three times as long as the length of the entire valve. 

 Many of the most marked modifications in shape assumed by this species have been care- 

 fully represented in my plate, and these will suffice to show how large a range of variation 

 must necessarily be given so as not to violate the laws of nature. Beautifully perfect 

 interiors of both valves are not rare in certain localities. On the concave surface of the 

 ventral one two elongated contiguous dendritic adductor or occlusor impressions project at 

 times considerably above the level of the valve, and immediately under, but outside, of 

 these may be seen the two large longitudinally striated subquadrate impressions, attributa- 

 ble to the divaricator muscle, while a glance at my figures of this and the corresponding- 

 valve of P. semireticul edits will show the difference in position occupied by these muscles 

 in the two species. The occlusors in the last-named shell are almost upon a level and 

 longitudinally parallel with the divaricator, while in P. longispinus the divaricators 

 commence only at or close to the base of the occlusors. A difference in the arrange- 

 ment of these muscles occurs likewise in P. punctatus, and denotes that the three species 

 might be distinguished alone by the details connected with their interior arrangements. 

 In the interior of the dorsal valve the cardinal process is proportionally large and trilobed. 

 under which a median longitudinal ridge extends to a little more than half the length of 

 the valve, and becomes much elevated and thickened towards its extremity ; on either side 

 may be seen a pair of dendritic scars formed by the adductor or occlusor muscle ; the 

 reniform impressions are also well defined and often much raised, and the surface of the 

 valve is covered near its margin with numerous spinulose asperities ; minute canals tra- 

 versing the valves are also clearly visible in the shape of punctures, especially upon speci- 

 mens that have been slightly weathered. 



In the ' Geology of Yorkshire/ Prof. Phillips figures two variations in shape of the shell 

 under description by the name of setose. ; his fig. 9 (15 of my plate) is a large, but not 

 unusual, shape of P. longispinus, while his fig. 17 (1G of my plate) is a rarer condition of 



