156 
THE AVONIAN OF THE AVON GORGE 
Hinge line straight, and less than the greatest width of the 
shell. 
A triangular area, with central delthyrium, in each valve. 
Ornament: narrow, close-set radial ribs which usually 
bear short procumbent spines. 
Outer shell-layer strongly punctate, especially on the ribs. 
Teeth and cardinal process well developed. 
Muscular scars deeply impressed and surrounded by a 
thickened margin. 
Distinctive characters : 
Schizophoria 
Pedicle valve flattened ; the muscular scars, which 
occupy a relatively small area, are narrow and flabelli- 
form and are separated by a strong ridge. 
Brachial valve strongly convex, often globose ; 
the two adductor-scars which lie on the same side of 
the middle line are not sharply marked off from each 
other, and the adductors on one side are separated from 
those on the other by a broad central plateau. 
Figures: Dav. Plates 29 and 30 (OrtJiis resupinata). 
Rhipidomdla 
Both valves flattened and approximately equal. 
The muscular scars of the pedicle valve cover a very 
considerable portion of the whole interior area of the 
valve. 
In the brachial valve, the two adductor-scars which lie 
on the same side of the middle line are sharply marked 
off from each other by a ridge which runs out, perpen- 
dicularly, from the mesial septum. 
Figures: Dav. Plate 30 (Orthis Michelini). 
Note : — The essential distinction between the above two genera is the 
very large size of the muscular scars in the pedicle valve of BMpidomdla. 
But, although this character will always serve to distinguish a typical 
specimen of Orthis Michelini from a typical specimen of Orthis resupiTiata, 
yet, in the case of specimens which are intermediate in outward form, the 
degree of development of the muscular scars is also intermediate, and the 
separation of the two genera becomes impossible. 
