CARBONICOLA AQUILINA. 71 



deep and smooth ; the hinder gradually becomes more shallow, and is rough and 

 subdivided by numerous irregularly branching lines. The anterior accessory 

 pedal scars are small, pit-like, and situated immediately behind the posterior- 

 superior angle of the anterior adductor. 



The posterior-adductor scars are shallow, somewhat broader posteriorly than 

 anteriorly, situated within the posterior slope close to the hinge-line, and not far 

 from the posterior end. The position of both adductor muscles is often indicated 

 on the exterior of the shell by marked depressions due to muscular strain. 



Casts indicate a longitudinal groove between the umbones and the hinge-edge, 

 PI. X, figs. 19 a, 20 a, 21 a, and a broad, deep excavation anterior to the umbones 

 for the hinge-locking teeth, crossing which the line representing the junction of 

 the two valves becomes sinuously curved, being deviated from before backwards, 

 first to the right and then to the left, demonstrating the presence of cardinal 

 teeth in the valves, of which that in the left is slightly anterior to that in the right. 

 There is always posteriorly a raised central ridge corresponding to the interval 

 between the hinge-plates, which shows that the " posterior elongate lamellar 

 teeth " of M'Coy and Amalizky are absent. The surface of casts is marked by 

 broad, shallow, eccentric sulci, and often with obscure radiating lines. 



Dimensions : Antero-posteriorly. Dorso-ventrally. Laterally. 



PI. IX, fig. 6 . 45 mm. 18 mm. 14 mm. 



PI. X, fig. 5 . 40 mm. 16 mm. 10 mm. 



PI. IX, fig. 1 . 30 mm. 15 mm. 10 mm. 



Observations. — This species is by far the most common form in British coal- 

 measures, both in its vertical and horizontal range, and it also occurs in many 

 localities abroad, in Belgium and Germany, a fact which is well attested by the 

 list of synonyms. The shell is very variable indeed in shape, size, and also in the 

 strength of the surface strias ; but I have not been able to convince myself of the 

 necessity of giving varietal names, from the study of large numbers of shells 

 belonging to this species from the same and different beds. I find that all forms 

 are met with in the beds at Whitley, Northumberland, the Hard Mine, North 

 Staffordshire, and the Grain's, Gubbin, and Brooch Ironstones, South Staffordshire, 

 showing, I think, by the presence of all the extreme forms and complete series 

 of intermediate ones, that they were all fertile inter se. 



The hinge characters appear to be constant, and to bear out Professor King's 

 observations {op. supra cit.), and I have met with no variation except that of 

 degree in the form of the hinge. 



It is important to note that abroad Ludwig, Amalizky, and others have mistaken 

 the sharp superior edge of the posterior part of the hinge-plate with the groove 



