CARBONICOLA AQUILINA. fli 
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, 
Pl. 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. 
PIPEX fis. 6 2 4dimm. 18 mm. 14 mm. 
Rex ene 5) ©) 4.0i mm: 16 mm. — 10 mm. 
Pixie. 0 30mm: 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. ‘he shell is very variable indeed in shape, size, and also in the 
strength of the surface striz; 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 
