372 CLYPEUS 



posteriorly, anal valley narrow, acutely lanceolate, with vertical walls, extending from the 

 disc to the border ; base concave, mouth-opening sub-central, peristome surrounded by 

 five prominent oral lobes. 



Dimensions.— Large specimen from the Forest Marble. Antero-posterior diameter, 

 three inches and a quarter ; breadth, three inches and three tenths. 



Average-sized specimen from the Great Oolite. Height, eight tenths of an inch ; antero- 

 posterior diameter, two inches and a quarter; breadth, two inches and three twentieths. 



Description. — This beautiful Clypeus was first found by my esteemed friend 

 S. P. Woodward, Esq., in the Great Oolite near Cirencester, and was referred by him to 

 Clypeus Solodurinus, Ag. In this opinion I formerly concurred, and described it under 

 that name in my memoir on the ' Cassidulidse of the Oolites,' already referred to in the 

 synonyms of this species. Having discovered a marly vein in the Great Oolite which 

 contained a number of specimens, I had an opportunity of studying this urchin in 

 different stages of development ; from this examination I ascertained that our original 

 determination could not be maintained, as the three specific characters insisted 

 upon by M. Agassiz, namely, the angular and truncated form of the posterior border, 

 the nearer approximation of the vent to the summit, and the possession of a very thick 

 test {" Le test est assez epais et reconvert d'une granulation assez uniforme sur toutes les 

 parties intactes"*), were characters which did not hold good in the suite of specimens 

 collected. I therefore determined to describe it as a distinct species, associating with 

 it a name most justly esteemed by all physiologists. 



Clypeus Mulleri has an oval form (fig. 2 a) ; rounded before, and slightly trun- 

 cated behind ; the test is very thin, and on that account is not often well preserved, 

 the upper surface is flat and much depressed, it slopes more towards the posterior than 

 the anterior border ; the marginal fold is rounded, and the sides are tumid in proportion 

 to the height of the test (fig. 2 c). 



The ambulacral areas are narrowly lanceolate, nearly of equal width, and closely 

 approximated around the disc ; the dorsal portions of the poriferous zones are widely petaloid, 

 each petal has an elegant leaf-like form, being narrow below, expanded in the middle, and 

 lanceolate above ; for a short distance above the border, the pores forming a pair lie close 

 together (figs. 1 and 3), then gradually become wider apart, until they attain their maximum 

 separation in the middle of the dorsal surface ; from this point they again gradually 

 approximate until they lie close together at the apex. The form of the ambulacral petals 

 (fig. 2 c) is one of the diagnostic characters of this species when compared with Clypeus 

 Plotii ; in the latter the separation of the pores takes place much nearer the border than 

 in Clypeus Mulleri ; they likewise taper more towards the apex, and form a much more 

 graceful figure on the upper surface of the shell than they do in that species. 



• ' Ecbinoderraes Fossilles de la Suisse,' prem. partie, p. 35. 



