330 J. W. DAYIS ON" THE GENUS PLEURACANTHUS, AGASS. 



8. Pleueacanthtjs eobijstus, sp. nov. PL XII. fig. 5. 



Spine. Length 3*5 to 4*5 inches. Breadth, about mid-length, -4 

 of an inch. JFrom the middle the diameter of the spine becomes 

 smaller in each direction ; towards the apical extremity it contracts 

 rapidly, and terminates in an obtuse point ; the base is reduced to 

 three fourths the largest diameter. The spine is straight along the 

 dorsal side ; the opposite one slightly curved. The lateral and an- 

 terior surfaces are covered with fine but very decided longitudinal 

 furrows, numerous towards the base, and disappearing, without 

 anastomosis, towards the apex. The general form of the spine in 

 section (fig. 7 a, h) is rotund, the lateral faces being produced out- 

 wardly so as to meet the more depressed curvature of the dorsal 

 aspect. The dorsal surface is wide, embracing nearly one third of 

 the entire circumference of the spine. It is produced so as to form 

 a large median ridge ; and along the centre of this there are a number 



Fig. 7. — Pleuracantlius rohustus, Davis. 

 a b 



Si 



a. Section of the spine near the base. h. Section nearer the point. 

 c. Denticles, enlarged. 



of small punctures, which occasionally coalesce and form a slight 

 groove. On each side of the median ridge is a proportionally deep 

 furrow. The angles formed by the outer edges of these furrows 

 and the sides of the spine are armed with a series of large closely-set 

 acuminate denticles (fig. 7 c). They extend from the surface of the 

 spine •! of an inch, being nearly one half the diameter of the spine 

 they are very strongly implanted, and recurved towards the base 

 The outer surface of the denticle, i. e. the one forming the largest 

 curve, is produced in the form of a minute carina or keel. There 

 are about twenty denticles, extending rather less than one half the 

 length of the posterior face. The internal cavity is round, and, 

 except at the base, is comparatively small ; it is in the centre of the 

 spine. The walls of the cavity are thin where it has been imbedded 

 in the muscles of the flesh ; they gradually gain in thickness and 

 strength until the cavity ends in a point about 1 inch from the distal 

 end, the remaining part being solid. 



The preceding species, viz. PleuracantJius robustus, alatus, and 

 altemidentatus, possess features in which, speaking broadly, they are 

 somewhat similar to each other. "Whilst, however, they possess this- 



