644. BRITISH PALAOZOIC FOSSILS. [Pisces. 
side shortest, slightly convex; crown flattened, slightly convex, or slightly concave in the middle; the lateral 
sides most prominent, often somewhat reflected, rounded ; surface granulo-punctate by the ends of the medul- 
lary canals, averaging eleven punctures in one line, as in P. porosus. Average length seven lines; width of 
posterior end six and half lines ; width of anterior end five lines. 
It is possible that this may be the median row of teeth of Psammodus porosus, with which it agrees- in 
surface, differing principally in its smaller size, symmetrical sides, and regular trapezoidal form. I have, how- 
ever, seen one large tooth of P. porosus (in the University collection) having a close approximation to the 
characteristic shape of the P. cornutus. Specimens have been found by Captain Jones arranged in lines, the 
convex edge of one fitting into the concave one of the other. 
Position and Locality—Not uncommon in the carboniferous limestone of Armagh; rare in the dark 
lower limestone of Lowick, Northumberland. 
PSAMMODUS RUGOSUS (AJ.) 
Ref. and Syn. = P. rugosus Ag. P. F. t. 14. fig. 17 and 14; t. 19. f. 14; +P. porosus id. ib. t. 13. 
Desc. Var. rugosus.—Oblong, nearly square or irregularly trapezoidal in form; the square, or those having 
the length and breadth equal, rarest ; most specimens about one-fourth wider than long, the long sides being 
either parallel, or one slightly concave and the other convex, in the latter case, the convex long side is shorter 
than the coneave one; these long sides are usually bevelled obliquely on the under side for articulation, the 
short straight lateral sides are always rudely vertical ; the surface is nearly level in the middle, but generally 
raised towards the short sides, which are usually the most prominent parts of the tooth, one being usually 
more prominent than the other, and frequently exhibiting a boss-like protuberance near one of its corners. 
Entire surface covered with close, small, rugged, flexuous ridges, averaging three to four in two lines, but 
varying from three to eight in the same space, and on specimens of the same size; the wrinkles run parallel 
to the long bevelled sides of the oblong teeth from one of the short sides to the other; the wrinkles have 
sometimes only one row of pores opening on them, and sometimes as much as three or four rows; in many 
of the specimens, but not all, there is a very much larger, rude, irregular plication of the bevelled, articular 
sides, which extends at variable distances across the tooth. Length varying from two inches to three and half ; 
the width varying from one inch to upwards of two; the depth of root and crown together of average speci- 
mens being nine lines. 
Var. porosus.—General size and shape the same as in the Var. rugosus, but the surface generally rather 
more even (though not universally so), and entirely or nearly destitute of the wrinkles of the surface of that 
variety, being regularly and uniformly covered with a coarse granulo-punctation, formed by the openings of the 
medullary tubes. 
I have little doubt that Captain Jones is right in suggesting the identity of the species P. rugosus and 
P. porosus of Agassiz, and I here describe them only as recognisable varieties; the var. rugosus seems the true 
type, but to lose the minute wrinkling of the surface by wear; some of the most rugged specimens having all 
the prominent parts worn smooth, and presenting the regularly punctate appearance of the P. porosus. Speci- 
mens illustrating this fact unusually well are in the Collection. On the other hand, large uniformly porous 
specimens of the P. porosus shew in occasional hollow places the characteristic rugation of the var. rugosus ; 
and a remarkable intermediate specimen of this kind is also in the Collection. 
Position and Locality.—V ar. rugosus, very common in the red lower carboniferous limestone of Armagh ; 
rare in the dark lower carboniferous limestone of Arnside, Kendal. 
Var. porosus.—Rare in the lower carboniferous limestone of Lowick, Northumberland; common in the 
carboniferous limestone of Armagh; not uncommon in the carboniferous limestone of Arnside, Kendal. 
