CLIiNOPISTlIA ABBREVIATA. 447 



lous, and the internal aspect of the umbo is excavated, probably for an internal 

 ligament. 



Exterior. — The surface is ornamented with fine concentric striae, and towards 

 the lower border obsolete folds may be seen. The microscope reveals numerous 

 fine radiating strias, especially over the anterior and middle thirds of the valve. 

 Shell thin. 



Dimensions. — PI. L, fig. 17, measures — 



Antero-posteriorly . . . .25 mm. 



Dorso-ventrally (at centre of hinge-line) . . 20 mm. 



From side to side (estimated) . . .10 mm. 



■Localitj/ — The Fourlaws Limestone at the Coomb Qaarry, Northumberland. 



Observations. — The discovery of G. abbreviata in Great Britain is due to the 

 careful work of Mr. J. Dunn, of Redesdale, to whom also we owe the discovery of 

 another species of the genus, (/. parvuJa, at the same locality. 



The specimen, PI. L, fig. 17, which is the only one which has yet been 

 obtained, has both valves present, but unfortunately the right has its hinge-line 

 and umbo covered by the left valve, and the latter is slightly incomplete behind, 

 so that the curious and characteristic boss, posterior to the umbo, is not seen. 



Described under the name Solemya by de Ryckholt, de Koninck referred the 

 species to the genus Glinopistlia of Meek and Wortlien, but did not note the 

 radiating lines which show the near affinity of the genus to Solenomya. 



De Koninck described another species, G. lata, which he considered distinct 

 from G. abbreviata ; but I strongly suspect the two to be varieties of the same shell. 

 The differential diao^nosis inwen is as follows in his observations on G. abbreviata : 

 — " Cette espece a beaucoup de ressemblance avec la G. lata ; elle en differe par sa 

 forme plus allongee et plus ovale, par sa taille plus faible et surtout par I'absence 

 des fines cotes concentriques et regulieres dont la surface des valves de la G. lata 

 est ornee." These conditions probably depend on the size of the shell and its 

 conditions of growth. Both species are said to occur at Tournai, G. abbreviata 

 being stated to be very rare, and (7. lata still rarer. 



G. abbreviata is not very unlike Meek and Worthen's species G. radiata, var. 

 Isevis, which, like my specimen, possesses, as the name indicates, radiating strias. 

 These authors describe their shell as " having obscure radiating marks near the 

 ventral margin, which are nearly always defined on internal casts." This species 

 seems to be distinct from G. abbreviata, as its contour has not the simple con- 

 tinuous sweep of the latter, but is interrupted about the centre of the lower 

 border by a contraction or sinuosity. G. abbreviata, is easily distinguished from 

 G. parimla by its obliquely ovate, convex form, the latter species being oblong and 

 compressed, and never attaining to one fourth the size. It is to be noted that 

 de Koninck draws attention to the fact that his figures of G. lata have not the 



