278 MICRASTER 



MiCRASTER BREViPORUs, AgassiZy 1840. PL LXII a, fig. 3 a — h. 



MicEASTEE BREViPOEUs, Agassiz. Cat. Ectyp. Foss. Mus. Neo., p. 2, 1840. 



— — Agassiz et Besor. Cat. raisonn6 des Echinides, p. 130, 



1847. 



— — ■ (TOrbigny. Prodrome, t. ii, p. 270, 1850. 

 ^- — Sorignet. Oursins de I'Eure, p. 62, 1850. 



— Leskei (TOrbigny. Paleont. Fran^aise Ter. Cr^taces, tora. vi, 



p. 215, pi. 869, 1853. 



— Besor. Synopsis des Echinides Foss., p. 366, 1858. 



— — Coquand. Synopsis des Foss. des Charentes, p. 134, 1860. 



— — Cotteau. Cat. des Echinid. de I'Aube, p. 34, 1865. 

 MiCRASTER BREViPOECS, Egbert. Comp. rend, de I'lnstitut. 25 Juin, 1866. 



— — Schliinbach,. Beitrag. des Griinsandes von Rothenfelde, 



p. 14, 1869. 



— — Be Loriol. Echin. Helv. Echinides Cretacees de la Suisse, 



2nle partie, p. 369, pi. xxxi, fig. 5, 1873. 



Diagnosis. — Test largely cordiform, rounded and grooved before, contracted behind 

 and truncated at the posterior border. Upper surface uniformly convex, declining a little 

 more anteriorly than posteriorly, with a prominent carina between the apical disc and 

 posterior border ; base convex, ambitus slightly undulated, rounded, and inflated. 



Dimensions. — Antero-posterior diameter 1-^ inch ; transverse diameter 1 ^-o inch ; 

 height l^ inch. 



Description. — The test of this Urchin has a cordiform shape ; it is rounded and slightly 

 grooved anteriorly, and tapers gently to the posterior border, which is truncated down- 

 wards and a little inwards. The upper surface is depressed and uniformly convex, and 

 declines a little more to the anterior than the posterior border ; a prominent carina more 

 or less elevated in different individuals extends backwards from the apical disc to the 

 summit of the periprocte, and divides into two ridges, which descend on each side of the 

 vent towards the lower part of the posterior border, where they disappear near the 

 fasciole. 



The ambulacra are short and straight, very unequal in length, and slightly depressed. 

 The anterior single ambulacrum is lodged in a sulcus, which grooves the anterior border 

 and extends to the peristome ; its pairs of pores lie wide apart, and are nearly obsolete. 

 The anterior pair are large and divergent ; they occupy well-marked depressions, and 

 their poriferous zones consist, in one of my specimens, of 30 p^irs of holes, each having a 

 connecting groove between the holes. The posterior pair are slightly bent and directed 

 backwards, and are about two thirds the length of the anterior pair. The pores are 

 similar in structure, and there are 18 to 20 pairs in each zone. The ambulacral plates 

 are concave, smooth, and plain, those of the single area alone carrying miliary granules. 



