ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 915 



but it does not occur at Ardmillan also, as he stated, for the shell thus quoted 

 belongs to the other species here described under the name CI. Andersoni. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Genus Chonetes, Fischer de Waldheim. 



Chonetes advena, sp. nov. 



(Plate XXI, figs. 6-11.) 



Shell concavo-convex, transversely semielliptical, widest along hinge-line, twice 

 (or more than twice) as wide as long ; cardinal angles rectangular. Pedicle-valve 

 gently and quite uniformly convex ; beak small, acute, scarcely incurved, not 

 prominent nor swollen, but rising above hinge-line ; hinge-area triangular, narrow, 

 with several (4-5) oblique tubules on each side of beak running inwards through 

 substance of shell from upper 'edge of hinge-area ; interior of valve with pair of sub- 

 triangular pointed muscle-scars, in contact posteriorly but strongly diverging at 

 about 120° anteriorly, extending about one-third the length of valve ; each scar 

 obscurely and unequally divided lengthwise by weak median ridge, and separated 

 posteriorly by stronger shorter ridge. At the anterior points of the muscle-scars there 

 arise short wide vascular trunks, each of which divides into two divergent smaller 

 trunks, splitting up into numerous smaller vessels which run forwards subparallel 

 to the margin ; the rest of the interior is covered with elongated radial lines of 

 small pustules, most developed near the margins. Brachial valve" with broad widely 

 divergent subtriangular low crural plates (ending in fine points), fused at base so as 

 to form transverse cardinal plate, deeply and widely cleft in middle ; cardinal process 

 very small, of three (or four) very short thin lobes ; muscle-scars large, very faintly 

 impressed ; pair of thick septa slightly divergent, extending forwards for more than 

 half the length of valve from base of cardinal process, dying out anteriorly and 

 apparently dividing the contiguous large oval faint muscle-scars into outer and inner 

 portions ; rest of interior covered with radial lines of elongated pustules. 



Surface of shell ornamented with fine close thread-like radiating lines of which 

 every fourth or fifth one is slightly stronger than the others. 



Dimensions. — Width, 14-18 mm. ; length, 7-9 mm. 



Horizon. —Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — The peculiar tubules through the substance of the shell in the cardinal 

 region are characteristic of the group of shells which includes Chonetes, Chonostrophia, 

 Chonopectus, Chonetina, and Chonetella, and show in casts of this species as small 

 rods, but they do not seem to correspond with external spines, as there are no 

 traces of the latter in any of the Girvan specimens. It is doubtful if the so-called 

 septa or ridges in the brachial valve are homologous with the true " brachial ridges " 



TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 131 



