Tertiary.'] PALEONTOLOGY OF VICTORIA. [Echinodermata. 



PLATES LXVL, FIGS. 1 AND 2, AND LXVIL, FIG. 1. 

 PERICOSMUS NELSONI (McCoy). 



DESCRIPTION. Ovato-cordate, greatest width about the middle ; obtusely and 

 broadly rounded in front, with a very shallow, wide, slightly indicated sinus, gradually 

 narrowing to the posterior end, which has a subtrigonal or rhomboidal slightly 

 hollowed truncation, the dorsal margin of which overhangs or projects more back- 

 wards than the ventral broad side ; the anal opening being a little above the middle, 

 vertically ovate, in depression j margin very obtusely rounded ; lower side moderately 

 convex, with a very prominent lip projecting forwards over the lunate mouth, which 

 is rather more than its own width from the anterior edge; greatest depth and 

 projection of upper and under surface a little behind the middle of the length ; 

 upper surface tumid, with an obtusely rounded prominent ridge from a little 

 behind the apex to posterior end, the surface sloping on each side with moderate 

 convexity to the obtusely rounded lateral margins, and abruptly rounded 

 in front from a point at the junction of the ambulacra much in advance of 

 the middle. Anterior ambulacral space smooth, flat, with a very slightly marked 

 depression towards the front margin, the ambulacral pairs of pores very small, 

 close, simple, about 6 in the space of 6 lines near the middle ; two lateral pairs of 

 ambulacra very slightly depressed in the petaloid part, anterior lateral pair make 

 an angle of about 130 with each other, and about 70 with posterior ones ; 14 pairs 

 of pores in 6 lines at middle of petal. Peripetalous fasciole distinct posteriorly 

 round the posterior pair of ambulacra, re-entering angularly between them and the 

 anterior lateral pair, along the posterior edge of which it extends outwards and 

 downwards on each side, in one specimen ending near end of petalloid part of 

 ambulacra, in another one continuing to join the lateral fasciole. Lateral fasciole 

 strongly marked, slightly interrupted or irregular along the sides, broad and distinct 

 on the front, making a slight downward wave at the margin in front of the mouth, 

 and making a deeper downward wave under the anal opening on the posterior 

 truncation. Tuberculation nearly uniform over the whole of the upper surface 

 (averaging 24 in a space of 6 lines), except along the anterior ambulacral space, 

 which is nearly smooth, and not being smaller nor more crowded within the 

 peripetalous fasciole. The ambulacra are so distinct near mouth as to form a 

 slightly defined floscelle. The tuberculation is larger on the lower than on the 

 upper side, largest on the middle, broad, ovate, convex plastron, seeming to diverge 

 in rows from an impressed midline, from the lip to a prominence under the posterior 

 truncation ; the course of the two posterior lateral ambulacra being marked by two 

 broad, nearly smooth bands, minutely granulated, with very few primary tubercles 

 irregularly scattered on them. Spines on trader side slender, striated longitudinally ; 

 about 4 lines long. The four genital pores are very large, rounded, and close 

 together at the apex. 



Length, usually 4 inches ; in proportion thereto, taking length as 100, greatest 

 width about middle, T Vs ; apex from front margin, -$ ; depth, ^ 5 2 5 ; length of 

 depressed portion of anterior lateral ambulacra, -^fa ; greatest width of ditto, T 7 ; 

 length of posterior lateral ambulacra, -fifc j width of ditto, -^ j depth of notch 

 in front margin, T | 7 . 



DEC. vn. 17 ~ C 



