OKDOVTCIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 839 



This corresponds to the shell called inflata, var. retrorsa, by Salter.* The internal 

 characters of the brachial valve are precisely those figured by Davidson in 0. porcata 

 (op. cit., iii, pi. xxxi, fig. 15) ; it also agrees with the specimen from Coniston figured 

 by M'Coy t under this name. A comparison with the original types proves this fact. 



With regard to external characters, the ribbing is identical with the Coniston 

 type, but M'Coy was incorrect in stating that the ribs only increase by division, for 

 they also increase by intercalation, and only occasionally bifurcate at one-third to 

 one-half their length in the pedicle-valve, while in the brachial valve they appear to 

 increase mainly by bifurcation at one-fourth to one-third their length and often 

 again at two-thirds, and sometimes close to the margin. The interspaces between 

 the ribs are subequal, rounded, and usually narrower than the ribs themselves. The 

 primary ribs number about 20-25 and continue right through to the margin without 

 increasing in size, and are regular and equidistant, but as a result of the intercalation 

 or division of other ribs there are usually about 75-80 on the margin. All the ribs 

 of whatever length are uniform and equal in size on the margin, and are straight, 

 except towards the cardinal angles of the brachial valve, where they curve back 

 slightly. There is considerable variation in the amount of convexity of the brachial 

 valve, but the shells are always strongly resupinate, and between the large swollen 

 and the gently convex condition all intermediate stages can be found. The pedicle- 

 valve likewise varies in the degree of concavity or flatness. 



In the brachial valve it should be remarked that the sides of the oval or 

 lanceolate cardinal process are obliquely channelled by 3-4 distinct grooves, and this 

 feature is also well seen in the type-specimen of Portlock's 0. grandis, \ which 

 Davidson (op. cit., pi. xxxi, fig. 19) refers to O. porcata. With regard to the muscle- 

 scars, the anterior adductor scars are roughened and smaller than the posterior 

 ones, as in M'Coy's Coniston types. 



In the interior of the pedicle-valve the muscle-scar is subpentagonal, well circum- 

 scribed, but not deeply sunken : it is weakly emarginate in front, and each half 

 seems composed of two unequal elements side by side, this feature being indicated 

 by the bilobation of the lateral margins. • In this respect the scar is distinguished 

 from that of 0. fiabellulum. 



The original specimen of 0. porcata on which the species was founded by M'Coy § 

 came from Portraine, but the description given by him of the ribbing does not apply 

 well to the shells from Coniston subsequently figured by him under the same name, 

 for he states that "the ridges [ = ribs] are angular, of unequal length, and frequently 

 branch into three or four at once, but alternating with these are some quite simple 

 from beak to margin and others dividing into two." The Tyrone shell described by 



" Salter, Journ. Geol. Soc, vol. i, 1845, name in table facing p. 21 ; id., Mem. Geol. Surv., vol. ii, 1848, p. 373, 

 pi. xxvii, figs. 3, 4. 



t M'Coy, Syn. Brit. Pal. Foss. JVoodw. Mus., 223, pi. iH, figs. 41, 42. 

 \ Portlock, Geol. Rep. Londond., p. 452, pi. xxxii, fig. 25. 

 § M'Coy, Syn. Silur. Foss. Ireland, p. 32, pi. iii, fig. 14. 



