Vol. 55.] DE. A. W. EOAVE ON THE GENUS MICRASTEE. 527 



fig. 3, and PI. XXXYIII, fig. 4, for the ambulacrum and peristome 

 of the characteristic passage-form. 



If we can find certain features in these small Micraster Leskei- 

 forms, which are not characters of immaturity, and differ from other 

 equally small examples of the M. 2^7'cecursor-ty^e, in the same zone, 

 then we shall have some grounds for defining a scheme of separation, 

 which will go far tovyards establishing M. LesTcei on a firm specific 

 basis. IS'ow the general characters of J/. Leskei are those which are 

 common to all low-zonal forms — namely, depressed and cuneiform 

 shape, anterior disc, mouth distant from border, shallow sulcus and 

 notch, deep ambulacra, and narrow fascicle. 



The essential features of the species are : — 



Interporiferous area. — Perfectly smooth, like that of 

 M. cor-hovis. A fine granulation is seen in the passage-form to 

 M. jyrcecursor, but never in the true M. Leskei. The inner row of 

 pores are often elongated, like those of M. cor-hovis CPl. XXXYI, 

 fig.2). 



Interporous ridge. — Either absent or weak. 



Single ambulacrum. — Much resembling that of 31. cor-hovis, 

 but the granulation is obviously closer and more abundant. Occa- 

 sionally, in perfectly typical examples, a trace of suturing is seen, 

 but the junction of the plates is, as a rule, not indicated. 

 (Pl.XXXyiI,fig. 2.) 



Peristome. — This is in the form of a smooth raised ring, with 

 the jointings of the anterior peristomal plates not indicated. Occa- 

 sionally, smooth nodular inflations are seen on the anterior segment, 

 the constrictions denoting the points at which the jointings would 

 appear in other species. This unbroken peristomal ring is found 

 only in M. cor-hovis and M. LesTcei ; the jointings are a characteristic 

 feature of all other Micrasters, and they become more marked as 

 one ascends the zones. The typical M. ZgsZ^ei-peristome is shown in 

 PL XXXYIII, fig. 3. 



Lab rum. — Always rudimentary and quite smooth. Sometimes 

 the smooth tip looks as if it were a separate piece which had been 

 stuck on ; but this is probably due to secondary deposition of car- 

 bonate of lime. 



Labral plate. — Triangular in shape, with the broad base at the 

 plastron, and tapering to a point at the labrum. Primary tubercles 

 very few and irregular, and no true linear arrangement even in the 

 largest examples. There are rarely more than six tubercles on the 

 plate, and often only two or three. PI. XXXYIIL fig. 1, from a 

 typical and well-preserved example, has only two — one at the top 

 and the other at the bottom of the plate. This rudimentary 

 tuberculation recalls M. cor-hovis. 



Size. — After comparing numerous examples, all of which exactly 

 correspond in their essential features, though derived from widely- 

 distant sections, the writer is forced to conclude that the true 

 M. Leshei is a small urchin, averaging about 34 mm. in length, and 

 rarely exceeding 40 mm. As soon as one passes outside the 

 40 mm.-range, the true M. Leslcei-iii'<iX\xvm begin to merge into those 

 of the passage-form between it and the Holaster planus-Lone, type of 



