LAMELLIBRANCHIATA. | DEVONIAN MOLLUSCA. 391 
specimen from the matrix, or it had been worn off from the soft sandstone in the carriage. The distinctness of 
this mesial fold in the mould of the entering valve would cause the specimen to be referred to the P. mesoloba, 
were it not for the distinct longitudinal striation; and as this is also the distinguishing character of the P. 
Christiani of de Koninck, I have little doubt of the identity of the species, allowance being made for difference 
in size, and the spines being rather more numerous than in the example figured by Koninck. 
Position and Locality.—Rare in the sandstone of Croyde. 
3rd Class) LAMELLIBRANCHIATA. See page 256. 
ist Ord. PLeuRoconcHA. See page 257. 
7th Family. AVICULIDA. See page 257. 
Genus. AVICULA. See page 257. 
AVICULA TEXTURATA (Phill.) 
Ref.—Pal. Foss. f. 87. 
Sp. Ch.—Obliquely ovate, tumid, evenly gibbous along the middle, posterior slope gradually compressed, 
obliquely subtruncate; anal cardinal angle obtuse ; respiratory angle broadly rounded ; anterior side small, acute, 
or rounded, very deeply divided from the body of the shell by an abrupt compression and a rounded sinus in the 
margin between it and the strongly convex ventral margin; entire surface covered with strong, prominent, 
concentric, narrow ridges, three or four times their thickness apart (four to five in a space of two lines at six 
lines from the beak in middle of the shell), the interspaces cancellated by radiating ridges of about the same 
thickness as the concentric ones, but rather closer (five to six in same space and part of the shell as above 
mentioned), the points of intersection forming tubercles. Average length from anterior end to respiratory angle 
one inch three lines, proportional length of hinge-line from beak to anal angle ;;, length of anterior side ;;, 
width from beak at right angles to hinge-line “, width of posterior end at right angles to end of hinge-line =, 
depth of left valve at one-third the length from the beak jj. 
Position and Locality.—Rare in the Devonian limestone of Plymouth ; rare in the hard Devonian schists 
of Linton, North Devon. 
Subgenus. PTERONITES (/*Coy). 1844. 
Ref.—Carb. Foss. Irel. 
Gen. Char.—Subtriangular, depressed, hinge-line as long as the shell ; beaks terminal or nearly so, 
forming a very narrow, obtusely-pointed, anterior end, from which the ventral margin extends uninterruptedly to 
the broad posterior end; left valve most convex ; internally a very small tooth under the beak of the right 
valve, and a very slender, posterior, lateral tooth close to the hinge-line the whole length; no other teeth or 
muscular impressions visible. 
Probably a subgenus of Avicula. Differs from Pterinea in the thinness of the shell, want of the 
ligamentary facet between the cardinal margins, absence of the hinge-teeth, and strong anterior adductor 
impression. Differs from Avicula in general form and want of a defined anterior lobe or byssiferous sinus, 
and defined posterior wing ; the nearly terminal beaks giving the narrow pointed character of the anterior end. 
Myalina of Koninck is described as equivalve, and as having a broad, striated, ligamentary facet. 
