528 DR. A. W. EOWE ON THE GENUS MICEASTER. [Aug. 1 899, 



Micraster prceeursor, which has the ' sutured ' interporiferous area, 

 and the peristome with jointed ])lates. 



Variations. — These are merely confined to shape, and the chief 

 variants are shown on PL XXXV, line i, Nos. 1' to 5', embracing 

 the depressed forma Normannice^th.e flat-arched forma, jylanidorsata, 

 the round-arched forma heo7iensis, the forma carinata, and the forma 

 gihbosa. It is unfortunate that these urchins were a little out of 

 focus when photographs of them were taken. Though they are 

 typical examples of the species, several of them are below the 

 average size. 



Young state. — More tumid, with shallower ambulacra and 

 ambital notch. 



Rostrum and carina. — Eostrum absent. Considering that 

 this is a very low-zonal form, it is curious that so many examples 

 should have a slight but definite degree of carination. This carina- 

 tion is greater than in any of the other forms of Micraster in the 

 Holaster planus-zone. It brings to mind the shape of M. Sanctoe- 

 Maurce, Gauthier. 



Sub-anal fasciole. — Always distinct, even in the smallest 

 examples, but decidedly thin. It is seen on PI. XXXVIII, fig. 1. 



Tubercles of plastron and anterior base. — Shown in 

 PI. XXXVIII, fig. 2, as a contrast to fig. 6 on the same plate, 

 which belongs to M. cor-hovis. For a description of the diff'erences 

 between these two species, the reader is referred to the section 

 dealing with M. cor-hovis (p. 522). 



Periplastronal are a. — Always finely granular (PI. XXXVIII, 

 fig. 1). See also note on the same area in the description of 

 M. cor-hovis (p. 521). 



Affinities and differences. — Apparently closely allied to 

 M. Sanctce-Maurce, Gauthier (which is found in the upper part of the 

 Inoceramus lahiatus-zone, in the Marne), diff'ering from it, however, in 

 that the French urchin is much smaller, the peristomal plates strongly- 

 jointed, the labrum smaller, and the carina more highly developed. 

 The distinctions between M. LesJcei and M. cor-hovis have been fully 

 discussed under the heading of the latter, and those between it and 

 its passage-forms will be set forth below. It is unnecessary to 

 compare M. LesJcei with M. cor-testudinarium, as there are no contact- 

 points between the two forms. 



It wiU simplify matters if, at this point, the features of the 

 passage-form are given. 



(3) Passage-form between Micraster Leskei and 

 M. prcecursor of the Holaster planus-zone. 



The general features of the test are the same as in all low-zonal 

 forms. These are set forth in the summary on p. 516. The essential 

 features are : — 



gize. — Anything between 35 and 70 mm. in length. 



Interporiferous area. — The ambulacrum is invariably of 



