Tertiary.] PALEONTOLOGY OF VICTORIA. [Echinodermata. 



PLATES LXIV. AND LXV. 

 PERICOSMUS GIGAS (McCoy). 



[Genus PERICOSMUS (AoAS.). (Sub-kingd. Radiata. Class Echinodermata. Order 

 Echinida. Fam. Spatangidae.) 



Gen. Char. Test heart shaped, broadly rounded and deeply notched in front ; slightly 

 narrowed and truncated behind to form a slightly overhanging, nearly vertical, slightly hollowed 

 surface for the anal opening ; ambulacral summit slightly excentric. Ambulacra : Anterior 

 ambulacrum in a deep sulcus, composed of two rows of very small, closely approximate, pairs of 

 pores ; lateral pairs of ambulacra elongate, petalliform, deeply excavated, anterior pair longer 

 than the posterior, each of two rows of pairs of pores, the outer pore of each pair larger than 

 the inner one, with which it is connected by a sulcus. Fascicles : A narrow peripetalous 

 fasciole, deeply indented between the two pairs of lateral ambulacral petals ; a marginal 

 narrow fasciole, not connected with the peripetalous fasciole, completely surrounding the test a 

 little above the lateral margins, descending in front, passing in front of the mouth on the under 

 side, and descending under the anal opening on the posterior end ; no subanal fasciole, nor 

 internal fasciole. Tubercles very numerous, small, subequal, erenulated and perforated, 

 surrounded by small miliary granules. Extinct. Tertiary. 



The species were formerly referred by Agassiz to Micraster, from which the absence of 

 subanal fascio], and presence of peripetalous fasciole distinguish Pericosmus, as well as the 

 disconnected lateral fasciole, unlike that of any other genus.] 



DESCRIPTION. Broad, ovate, greatest width a little behind the middle, very 

 deeply notched in front, slightly narrowed and subtruncate behind j margin narrow, 

 rounded except on the rhomboidal overhanging posterior space, on upper portion of 

 which the anal opening is placed ; upper surface moderately convex, sloping with 

 gradually less convexity to the margin, but rising into an obtuse ridge from vertex 

 to posterior end. Under side moderately convex. Mouth close to anterior notch, 

 transversely lunate, posterior lip prominent. Anterior ambulacrum sunk in a very 

 deep sulcus, deeply notching the front margin ; composed of two rows of distant, 

 small pairs of closely approximate, simple, slightly ovate or nearly round pores, 

 about 4 in the space of six lines at middle. Anterior lateral pair of ambulacra 

 forming with each other an angle of 140, arching gently forwards from the apical 

 system for three-fourths of their length and then giving a slight sigmoid bend 

 backwards ; about 8 pairs of pores in a space of six lines at middle ; posterior pair 

 of ambulacra straignter and making an angle of about 55 with each other. 

 Peripetalous fasciole narrow, distinct, commencing at about one-fifth the length of 

 the anterior ambulacral ridges from the front margin, then at about half their length 

 turning outwards nearly at right angles, curving forwards slightly at the end to 

 surround closely the end of each anterior lateral ambulacrum, along the posterior 

 side of which it runs, with several flexures and irregularities of depth, until at 

 about one-fourth the length of the ambulacrum from the vertex it turns backwards 

 at rather more than right angles, again turning at a slightly obtuse angle backwards 

 and outwards along the outer side of the posterior pair of ambulacra, curving closely 

 round their ends and then forwards and inwards along the inner margin of the 

 posterior ambulacra for almost the distal third of their length; and then arching 

 across the back. Lateral fasciole narrower and less distinct, starting from about the 

 middle of side a little above the margin, from a slight tubercular swelling of 

 posterior lateral interambulacral plate, turning downwards at the posterior end, 

 apparently terminating at edge of posterior truncation a little below the level of the 



[15] 



