DAVIDSON — SCOTTISH OARBONIPBROUS BRACHIOPODA, 



103 



and the name distorta should therefore be retained only as a varietal designa- 

 tion. 



In 18^3, Prof, de Koninck observed with justice that it was impossible to 

 distinguish the Silurian /S'/zc/;^. rhomboidalis ixom the Carboniferous St. analoga, 

 and he united the two, as well as several others, under a single denomination ; 

 and although the generality of palaeontologists have preferred retaining the two 

 as distinct species, they have also admitted that it was scarcely possible to 

 separate the Silurian, Devonian, and Carboniferous specimens. Both varieties 

 have been found in Scotland; while the typical form has sometimes attained an 

 inch and a-quarter in length by two and a-half in breadth, and still larger indi- 

 viduals have been obtained in England and in L'cland, In Stii'lingstiire the 

 typical variety is found in the Campsie main-limestone and Corrie Burn beds ; 

 while the variety distorta occoi's at Gare in Lanarkshire, at two Imndi-ed and 

 thirty-nine fathoms below the "Ell coal," and three hundred and forty-three at 

 Waygateshaw ; in Renfrewshii-e, at Davieland-quarry, near Thornliebank. 



Sub-genus Steeptoehynchus. King. 1849. 



The shells composing this sub-genus are closely related to Strophomena ; they 

 are usually semicircular, convex, or concavo-convex, and externally striated; 

 the ventral valve possessing a prolonged and oftentimes bent or twisted beak. 

 But a smgle species and some varieties occui- in Scottish Cai-boniferous strata. 



XXV. — Streptorhynchus crenistria. Phillips. PL i., figs. 16-25. 



Spirifera crenistria, Phillips' Geol. of Yorksliire, vol. ii., pi. ix., fig. 6, 1836. 

 Spirifera arachioidea, S. senilis, and S. radialis of Phillips ; Ortliis Kellii, 

 0. cylindrica, 0. caduca, 0. granulosa (?), and 0, Bechei, M'Coy ; 0. Sharpei^ 

 Portlock ; and 0. Portlockiana, Semenow, appear to be either synonyms or 

 simple varieties of Str. crenistria of Phillips, 



The shells of which this species is composed are more often compressed and 

 semicircular; the hinge-line straight, ana either a little shorter or slightly ex- 

 ceeding the width of the sheU at the prolonged acute cardinal angles. The 

 valves vary considerably in their curves and directions ; in some specimens they 

 are both convex or much compressed, and straight throughout, while in other 

 examples the dorsal one alone is convex, or flat to a certain distance, when it 

 becomes deflected, and assumes a lesser or greater degree of convexity ; the 

 ventral valve being in the same shell convex at, and to some distance from, the 

 beak, when it becomes slightly concave towards the margins, following the 

 curves of the opposite valve. The area in the dorsal valve is linear, while that 

 of the ventral one varies much in breadth and regularity on account of the beak 

 being sometimes tmsted more to the one than to the other side. The ventral 

 area is likewise divided by a triangular fissure, covered by a pseudo-deltidium. 

 Exteriorly the valves are ornamented with numerous strong radiating crenu- 

 lated striae, of unequal thickness, which increase in number towards the margin, 

 while the interspaces are similarly and partially occupied by one or more smaller 

 longitudinal striae. In the interior of the dorsal valve a strong hinge-tooth is 

 situated on either side at the base of the fissure, supported by small dental or 

 rostral plates. The muscular impressions form a saucer-shaped depression, 

 partially surrounded by a slightly elevated ridge ; the occlusor (A) occupies the 

 central portion, and forms two small elongated impressions, separated by a 

 slightly elevated mesial ridge, and on either side are the larger scars (R), appa- 

 rently composed of two parts, the anterior or central being due to the divarica 

 tor, while the other or outer one would be produced by the ventral adjustor? 

 In the interior of the dorsal valve, the cardinal process (to which were afiixed 

 the divaricator muscular fibres) is composed of two testaceous projections. 



