ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 931 



entering other valve, and holding 2 equal angular plications arising at about half 

 the length of valve ; anterior sides of sinus steep, with angular edges ; 2 or 3 short sub- 

 marginal angular plications of decreasing strength on each lateral lobe. Brachial 

 valve very convex, swollen, with low broad median fold arising some distance in 

 front of beak and composed of 3 coarse equal angular plications ; 2 or 3 similar ones 

 on each lateral lobe arising at about half the length of valve. Beak rather higher 

 than that of opposite valve. Interior of brachial valve with elongated narrow 

 spondylium nearly one-third length of valve, and thin median septum extending 

 in front of it. Interior of pedicle-valve with minute spondylium and short median 

 septum about one-third the length of valve. Surface of valves marked with con- 

 centric growth-lines. 



Dimensions. — Length, 875 mm. ; width, 9'50 mm. ; depth, 8'25 mm. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead ; (2) Balclatchie. 



Remarks. — This species differs from P. scotica by its fewer and coarser plications, 

 particularly on the fold and sinus. It seems more closely allied to " Camerella " 

 Tornquisti, Wiman,* from the Ordovician of the Baltic provinces. Our species is 

 dedicated to the late Professor Young of Glasgow. 



Genus Stricklandinia, Billings. 



Stricklandinia lens (Sowerby), var. 



(Plate XXII, fig. 48.) 



1883. Stricklandinia lens (Sowerby), Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 p. 165, pi. ix, figs. 4, 5 (non figs. 2, 3). 

 Non 1839. Atrypa lens, Sowerby, in Murchison's Silurian System, p. 637, pi. xxi, fig. 3. 

 Non 1866. Stricklandinia lens, Sowerby, Davidson, op. cit., vol. iii, p. 161, pi. xix, figs. 13-21. 



The transverse shells figured by Davidson as Str. lens from Woodland Point differ 

 from the typical subcircular Str. lens of English and Welsh localities principally by 

 their shape, being much wider than long. The beaks, moreover, of the valves are 

 nearly in contact, and there is no cardinal area exposed befow the beak of the pedicle- 

 valve as in the typical form. The fold and sinus are usually more sharply defined 

 from the lateral portions of the shell than in the type, but the internal characters 

 appear to be identical. We may observe some variation in the shape of the shell, for 

 stages between transversely elliptical and subcircular examples occur. The general 

 absence of radial plications and the more broadly rounded cardinal angles and shorter 

 hinge-line serve to distinguish this form from the Mulloch Hill species described 

 below as Strickl. mullochensis. But some subcircular specimens with faint radial 

 markings prove the intimate relationship of the various Girvan forms. 



Horizon. — Saugh Hill Group. 



Localities. — Woodland Point, Newlands, Shalloch Forge. 



* Wiman, Bull. Geol. Instit. Upsala, No. 10, vol. v, pt. ii, 1900, p.«175, pi. vii, figs. 22-25. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 133 



