ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 937 



Order TELOTREMATA. 



Genus Protorhyncha, Hall and Clarke. 



Protorhyncha nasuta (M'Coy). 



(Plate XXIII, figs. 20-23.) 



1852. Hemiihyris nasuta, M'Coy, Syn. Pal. Foss. Woodw. Mus., p. 203, pi. il, figs. 5, 5a. 



1868. Rhynchonella nasuta (M'Coy), Davidson, Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 173, 



pi. xxiii, figs. 19, 19a-fr. 

 1883. Rhynchonella nasuta (M'Coy), Davidson (pars), op. cit,, vol. v, Silur. Suppl., p. 160, pi. x, fig. 20 



(non fig. 2L). 



Shell longitudinally oval to subquadrate, either longer than wide or about as wide 

 as long ; cardinal angles and sides rounded ; anteriorly more or less produced into 

 rounded tongue and marginally sinuated ; unequally biconvex ; hinge-line curved. 

 Pedicle-valve less convex than brachial valve, but with more prominent higher im- 

 perforate (?) beak rising above hinge-line, incurved, with triangular hinge-area and 

 small triangular open delthyrium below it ; surface of pedicle-valve with broad 

 shallow Hat-bottomed sinus increasing in width anteriorly, and having steeply sloping 

 sides and angulated edges towards front margin ; floor of sinus occupied by 6-7 equal 

 regular ribs ; sides of sinus with usually one similar rib in posterior half but smooth 

 anteriorly; lateral portions of valve convex, covered with 12-16 somewhat smaller 

 and closer straight ribs ; interior of valve with short median septum in umbonal 

 region. Brachial valve more convex than opposite valve, with weak rounded median 

 fold on surface increasing in height and width anteriorly ; beak much smaller and 

 lower than that of pedicle-valve, obtuse, incurved ; surface of fold bearing 5-7 ribs, 

 and lateral lobes bearing 12—16 similar or slightly smaller ribs. 



Dimensions. — 



(1) (M'Coy's type.) (2) (3) (Davidson's figured specimen, 



pi. x, fig. 20.) 

 Ped. -valve. Brach. valve. Ped. -valve. Brach. valve. 



Length . . 29"0 26"0 c. 22'0 25*5 mm. 



Width . . 23'5 23*5 c. 21'0 c. 23'0 mm. 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Remarks. — This species appears to be related to the so-called Porambonites 

 ottaivaensis, Billings,* which Hall and Clarke t believed might be referred to either 

 the genus Protorhyncha or Orthorhynchula. Our shell seems to suggest its refer- 

 ence to Protorhyncha, for the hinge-line is not straight as it is in Orthorhynchula. 

 Atrypa dubia, Hall,| is chosen as the type of Protorhyncha, and this species occurs 

 in the Chazy Limestone, but Hall and Clarke (op. cit., pi. lvi, figs. 7-9) figure 

 another species (Pr. sequiradiata) from the Clinton Group which seems more like 



* Billings, Geol. Canada, Palxoz. Foss., vol. i, p. 140, figs. H7a-g. 



t Hall and Claekk, op. cit, Brach., ii, pp. 180, 181. 



J Hall, Palxont. New York, i, 1847, p. 21, pi. iv bis, fig. 5 



