522 DE. A. W. KOWE ON THE GENUS MICRASTER. [Allg. 1 899, 



In the case of Micraster LesTcei this band is never smooth, and 

 the contrast between it and the same area in M. cor-hovis is plainly 

 shown in PI. XXXYIII, fig. 1, and PI. XXXIX, figs. 2 & 3, 

 That the area is really smooth in M. cor-hovis may be seen in 

 PI. XXXYIII, fig. 6, which shows the junction of the plastron 

 and the periplastronal area. To Gen. C. F. Cockburn, who supplied 

 Forbes and Wright with so much material, belongs the credit of 

 this observation, and the writer agrees with him that this feature is 

 diagnostic of the species. 



Primary tubercles on base. — These are similar to those on 

 the plastron. At the anterior base they are very scattered, showing 

 up strongly on account of the feeble areolar circle, and the absence 

 of inter-areolar granulation, and they afford an additional aid in 

 making a determination between this species and M. Leskei. In 

 the latter the tubercles are crowded, the areolar circles are thick, 

 and the inter-areolar spaces are filled up with strong granulation. 

 Compare PI. XXXYIII, fig. 6 with fig. 2. In large specimens of 

 M. cor-hovis the tubercles on plastron and base appear conspicuously 

 small. 



Basal plates. — In all large and unworn specimens these form 

 a noteworthy feature, as they are set at slightly different angles, 

 and their sutures are very prominent. Owing to the varying aagle 

 of incidence of light, these plates give the base a curiously facetted 

 appearance. This feature is found in Holaster and Cardiaster, 

 and is merely a question of thinness of test. It is worth noting, 

 however, as it is rarely seen in the thick-tested forms of Micraster, 

 and is characteristic of the species under discussion. This feature 

 is beautifully brought out iu Wright's figure. 



Affinities and differences. — As will have been gathered 

 from the foregoing detailed description, this species has not a few 

 points in common with Hemiaster Morrisi. Those which it has in 

 common are the thin test, shallow sulcus and notch, mouth distant 

 from border, similar peristome and labrum, rudimentary tubercu- 

 lation of labral plate, smooth ambulacra with contracted ends in 

 posterior pair, elongated inner row of ambulacral pores, smooth 

 periplastronal area, and lateral bosses. The points of divergence, 

 on the other hand, are that in H. Morrisi we find a peripetalous 

 fascicle and no sub-anal fasciole, an accentuated carina, a jackward- 

 sloping posterior truncation, broader ambulacra, the anterior paired 

 ambulacra being equally pointed at the ends with the posterior pair; 

 the primary tubercles on the base are set in very narrow saucer- 

 shaped areolae, with raised edges, and the apical disc is much less 

 excentric anteriorly, and sometimes posterior. Though Hemiaster 

 belongs to a difi'erent genus, there are sufficient points in common 

 between it and M. cor-hovis to make the comparison of interest from 

 an evolutional standpoint, as the latter shows certain contact-points 

 with an earlier form. 



The comparison with M. Leskei is equally instructive, as it marks 

 the passage from the thin-tested Terehratulina gracilis-iorm. to the 

 thick-tested Holaster jplanus-iovm. The points of similarity are the 



