OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 249 
Famity. AvicuLips, Gray. 
24. MONOTIS, Bronn. 
1. M. erypuzorpes, Sov. 
Avicula grypheoides, Sow., 1829, Geol. Tr., 2nd ser., iii., p. 
119, No..-7. 
Avicula speluncaria, Quenst., 1835, Weigm. Arch., pl.i., f. 1, 
In Magnesian-limestone ; not rare. Humbleton, Tunstall, 
Silksworth, Ryhope, Dalton-le-Dale. 
This semi-flattened shell is very generally distributed in the 
fossiliferous bed of limestone, which is quarried in the above 
localities. The surface of the convex valve, which is generally 
imperfect, is covered with a few strong imbricated or granulated 
diverging ribs, between which there are others much finer and 
closer together. The flat valve is quite smooth, and has a deep 
triangular notch near the hinge-margin. 
As Mr. Sowerby’s name has the right of priority, we have 
adopted it, though that gentleman had subsequently applied the 
same name to another very different shell, probably an Inocera- 
mus, from the Green-sand ; see Geol. Tr. 2nd ser., vol. iv. 
25. AVICULA, Lamarck. 
1. A. antigua, Miinst. 
Avicula antiqua, Miinst., Goldf., Petrif., pl. 116, f. 7. 
In Magnesian-limestone ; not rare. Tunstall, Humbleton, 
Silksworth. 
This is the commonest and largest species of Avicula found in 
the limestone. The surface, when perfect, is covered with nu- 
merous, fine, elevated, lines of growth. In this species the pos- 
terlor extremity is broad, and slightly forked. 
2. A. KERATOPHAGA, Quenstedt. 
Avicula keratophaga, Goldf., Petrif., pl. 116, f. 6. 
In Magnesian-limestone ; not rare. Tunstall, Humbleton. 
This species is much smaller than the former. The posterior 
is narrower, more produced, and much forked. The anterior is 
small and pointed. The surface is finely striated, asin the A. 
antigua. 
