Tertiary.'] PALEONTOLOGY OF VICTORIA. \Echinodermata. 



PLATES LXVIL, FIG. 2, AND LXVIII. 

 PERICOSMUS COMPRESSUS (McCor). 



DESCRIPTION. Orbicular, a little wider than long, slightly truncated behind, 

 very deeply notched in front, depressed, moderately convex above, no distinct ridge 

 along posterior interainbulacrum, which is a little more convex than the others ; a 

 deep furrow with abruptly rounded sides extends from the apex to the very deep, 

 anterior, marginal, parallel-sided notch ; margin much compressed, narrow, except 

 on obliquely -overhanging, rhombic, posterior space which contains the anal opening 

 near its upper edge ; under-surface very flat ; central, tuberculated, plastron and 

 sides very slightly convex, broad, more finely granulated ; ambulacral spaces 

 slightly concave. Mouth half its width from margin of anterior notch. Apex a 

 little in front of centre ; anterior ambulacrum in the broad, deep, anterior sulcus ; 

 the two rows of pairs of small, nearly rounded, closely-approximated pores (about 6 

 pairs in a space of 3 lines a little above middle of length) ; two lateral pairs of 

 ambulacra nearly straight and nearly equal, the anterior scarcely exceeding the 

 others, very deeply excavated ; the anterior lateral pair make an angle of 

 120 with each other, the posterior pair make an angle of 70 with each other ; there 

 are about 18 pairs of pores in six lines at middle of four posterior ambulacra. 

 Fascioles: Peripetalous fasciole distinctly surrounding the two lateral pairs of 

 ambulacra, close to the tips of the depressed petalloid portions, and indented between 

 them across the posterior and two lateral interambulacra, but becoming indistinct 

 (in our specimens) after rounding the ends of the anterior pair, about half-way to 

 the anterior sulcus ; marginal fasciole very narrow, but prominent and distinct all 

 the way round a little above the margin (indistinct where dipping under anus in 

 our specimens). Tuberculation : Primary tubercles close and nearly uniform 

 above, about 27 in 6 lines ; about 12 in 6 lines on middle plastron, below j 

 largest, more scattered on sides until the curve of the margin is reached. Length, 

 3 inches 3 lines ; in proportion thereto, greatest width about middle, j^$ ; apex to 

 front margin at side of notch, -fo ; greatest depth, -f^- j length of anterior lateral 

 ambulacra, -fifo , greatest width of ditto, 6 j length of posterior ambulacra, T 2 

 depth of notch in front margin, 



From the general shape, size, and near equality of the four 

 posterior ambulacra and the very deep anterior notch, I should 

 have thought the Megalaster compressus of Prof. Duncan 

 (J. G. S., v. 33, p. 62 and 68) might have been identical with 

 this fossil ; but as Prof. Duncan forms a new genus of his form, 

 and repeats his remarks as to the absence of all fascicles, while 

 the peripetalous and marginal fascicles are unmistakable in ours, 

 which consequently belongs to the genus Pericosmus, I suppose 

 there can be no doubt of their distinctness ; our fossil has, also, a 

 convexity instead of a concavity between the two posterior 



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