THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 5D3 



The Craighead specimen consists of the left valve, probably an internal cast. It is 

 imperfect at the anterior end. The Woodland fossil specimen is crushed, but both 

 valves are present. The periostracum is preserved. 



I see no reason for separating the two specimens, nor do I see sufficient reason 

 to erect a new species, and therefore have referred both examples to Miller's 

 S. richmondense. 



Genus Modiolopsis, Hall, 1847. 



Modiolopsis, Hall, 1847, Pal. New York, vol. i. p. 157. 



Miller, 1889, N. Amer. Geol. and Pal, p. 489. 



Generic Characters. — Modioliform, inequivalve, inequilateral, transverse, with an 

 expanded and compressed posterior end. Umbones small, anterior. Cardinal teeth 

 short, oblique (Miller). Anterior adductor muscle scar deep circular, almost 

 marginal. Ligament external. No escutcheon or lunule. Surface with concentric 

 lines. 



Observations. — There are a number of closely related families of modioliform shells 

 in Ordovician rocks, Modiolopsis, Modiolodon, Modiomorpha, which differ from each 

 other in small details of hinge structure. It is difficult, in the absence of any details of 

 the hinge plate, to assign any species accurately to its genus. American specimens seem 

 to have been much better preserved than those found in Great Britain, and to have had 

 the essential details of structure better exposed. 



Modiolopsis expansa, Portlock, sp., 1843. (PI. II., fig. 21.) 

 Modiola expansa, Portlock, 1843, Geol Rep. Londonderry, p. 425, pi. xxxiii. fig. 6. 



Specific Characters. — Shell transverse, narrowed in front, expanded and compressed 

 posteriorly, carinate. The anterior end is narrow and produced, its border elliptical. 

 The inferior border curved, the posterior inferior angle bluntly rounded. The posterior 

 border straight and oblique, the postero-superior angle obtuse. The hinge line straight, 

 shorter than the inferior border. The umbones are carinate, small, incurved, placed 

 about the junction of the anterior and middle thirds of the valve. Passing down 

 obliquely from the umbo to the posterior inferior angle is a well-marked subangular 

 keel, dividing the shell into two triangles, both of which are flattened and compressed, 

 the posterior more rapidly than the anterior. No lunule or escutcheon. 



Interior. — Unknown. 



Exterior. — Surface covered with concentric lines and striae of growth. 



Dimensions. — Fig. 21, PI. II., a right valve, measures: antero-posteriorly, 

 44 mm. ; dorso-ventrally, 26 mm. 



Locality. — Scotland : Llandeilo, Craighead. Ireland : Desertcreat, Co. Tyrone. 



Observations. — Portlock states that he found two varieties of this species, one 



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