ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 939 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Remarks. — This shell must be referred to the genus Orthorhynchula, Hall and 

 Clarke,* on account of its straight hinge-line and the presence of a hinge-area. It 

 is not quite clear if the delthyrium is closed by plates or open. The first specimen 

 figured by Davidson (fig. 5) is an unusually thick and subglobose example, and his 

 fig. 6 represents the common type, the other being quite exceptional. We may 

 compare this species with O. Linneyi (James), f from the Lorraine Group, the only 

 described species of the genus ; this species also varies in its degree of globosity. 



Genus Rhynchotrema, Hall. 



Rhynchotrema Lapworthi (Davidson). 



(Plate XXIII, figs. 28-31.) 



1883. Rhynchonella Lapworthi, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 154, pi. x, 

 fig. 7. 



This species has 4 ribs on the low fold in the brachial valve, 3 in the better- 

 defined sinus in the opposite valve, and 3-4 on each lateral lobe, with 1 or 2 smaller 

 ones near the hinge and the lateral false areas. The ribs are all equal in size, 

 except the marginal ones bounding the sinus, which become rather higher anteriorly, 

 and the 1 or 2 cardinal ones which are smaller than the rest. Davidson's figure does 

 not make the ribs on the fold straight enough, nor the top of the fold sufficiently flat ; 

 the sides of the sinus are also not shown as steep as they are in reality. This species 

 much resembles Rh. insequivalve (Castelnau),J from the Trenton Formation of North 

 America. 



Some specimens of Rh. Lapivorthi are considerably more globose and less 

 transverse than the type, but cannot be separated specifically. 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Rhynchotrema shallochense (Davidson, emend.). 

 (Plate XXIV, figs. 1, 2.) 



1883. Rhynchonella shallockiensis, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 154, 

 pi. x, fig. 19. 



This is a badly founded and badly defined species, for undoubtedly the shell 

 which Davidson figured as the type (measuring 7 mm. long and 9 mm. wide) is an 

 immature individual, and on the same tablet there were mounted by him three larger 

 and more normally rhynchonelloid shells and one specimen referable to Platystrophia 

 biforata. The shell next in size to the type, and probably more advanced in age, is 



* Hall and Clarke, op. cit., Brach., ii, p. 181. 



t Nettelroth, Mem. Kentucky Geol. Surv., 1889, p. 41, pi. 34, figs. 7-18 ; Hall and Clarke, op. cit., p. 181, 

 pi. lvi, figs. 10-13, 19 ; Foerste, Bull. Denison Univ., vol. xvi, art. 2 (1910), p. 24, pi. iii, fig. 10. 

 \ Winchell and Schuchert, Palseont. Minnes., vol. iii, p. 459, pi. xxxiv, figs. 9-25. 

 TRANS. ROY. SOC EDIN., VOL. LI, PART IV (NO. 26). 134 



