204 CARBONIFEROUS LAMELLIBRANCHIATA. 



They are situated a little way posterior to the middle vertical diameter of the shell. 

 The escutcheon is divided into two equal longitudinal portions behind by the 

 thin margins of the edges of the valves, which are in contact and extended 

 upwards. It is bounded on either side by a well-marked, sharply denned ridge, 

 springing from the umbo (giving the latter the appearance of pointing back- 

 wards), which passes downwards and backwards to the lower angle of the 

 posterior extremity. 



Interior. — The anterior adductor muscle-scar is long and deep, situated low 

 down and near the margin. The posterior is situated just within the narrow 

 posterior end. The hinge-plate is in two portions, separated by a subumbonal 

 cartilage-cavity. The anterior teeth are larger than the posterior, and are 

 situated in the hinge-plate for a short distance anterior to the umbo. They 

 are only about six in number. The posterior are also about six, and do not 

 extend far behind the umbo. The pallial sinus is entire and remote from the 

 margin. 



Exterior. — The surface is ornamented with well-defined, regular, concentric 

 stria?, with here and there towards the ventral margin a deeper sulcus. These 

 become lost soon after passing over the ridge forming the border of the escutcheon, 

 and leave the immediate posterior extremity almost smooth. 

 Dimensions. — PI. XV, fig. 26, measures — 



Antero-posteriorly . . . .19 mm. 



Dorso-ventrally .... 12*5 mm. 



From side to side . . . .11 mm. 



Localities. — England : Redesdale Ironstone, Northumberland. Scotland : 

 Upper Limestone series; Gare, Orchard, Garngad Road, Glasgow; Lower 

 Limestone series ; Lingula Ironstone and first Kingshaw Limestone, Carluke ; 

 Campsie and Thornton in shale above the Hosie Limestone ; Eastbarns, Dunbar ; 

 Wilkieston, Fife, about the horizon of the Hurlet Limestone. 



Observations. — This species has always been referred to Nucula except by 

 d'Orbigny, who more correctly referred it to Leda, but upon what grounds it is 

 difficult to understand. Strangely enough also, Phillips, M'Coy, and R. Etheridge, 

 jun., have described the typically characteristic posterior end as anterior. Phillips's 

 description is very curt : " ovate, anterior side attenuated, obtuse ; surface neatly 

 striated across." It is clear which end he had in mind when he described it as 

 "attenuated." M'Coy states that the species has its "anterior side produced, 

 contracted, rounded;" and lays further emphasis on this feature in his remarks. 

 The specimen described by Mr. R. Etheridge, jun., as N. Youngi is evidently 

 only a very young example of Nuculana brevirostris, in which condition the 

 linear concentric markings are very finely shown, being comparatively much 

 better marked than in the adult state. His specimen was obtained from 



