BRITISH FOSSILS* 



7 



Description op the Plate. 



Fig. 1, var. y, dorsal view. Fig. 2, ventral surface. Fig. 3, profile. Fig. 4, diagram 

 of the dorsal structure. Fig. 5, apical disk of ovarian and ocular plates. Fig. 6, portion 

 of surface of a plate magnified. Fig. 7, arrangement of the pores in the ambulacral 

 avenues. 



Note on British Nucleolites allied to N. clunicularis. 



Under the generic appellation Nucleolites I include all the members of the family Cas- 

 sidulidcB (Agassiz) with petaloid ambulacra and a supra-marginal anus, and under the 

 subgenus Nucleolites those members of the genus so called, in which the anus is placed 

 high up within a deep sulcus. To the orbicular species of this division the generic term 

 ClypeiLS (KleinJ is restricted in the arrangement of Agassiz, who styles those only 

 Nucleolites in which the shape is more or less angularly subquadrate. He further dis- 

 tinguishes between the two sections by the presence of tubercles or prominences around 

 the mouth in the former, and their absence in the latter, a distinction which does not hold 

 good, since in Nucleolites lacunosus the oral prominences are more strongly marked than 

 in Clypeus Hugii. There is, moreover, a gradual progression of form from the almost 

 completely orbicular outline of Clypeus sinuatus to the ovate contour of Nucleolites 

 lacunosus. 



Though the name Clypeus was used by Klein before that of Nucleolites, the latter is 

 preferred because proposed as a definite generic appellation by Lamarck. 



Our British Nucleolites are all either oolitic or cretaceous. In the species of the older 

 secondary rocks we find a structure of the ambulacral pores which distinguishes them 

 easily on close examination, however nearly they may resemble at first glance, from 

 those of the upper secondaries. The former have the external series of pairs of pores in 

 each ambulacrum more or less widened and marked by conspicuous connecting furrows ; 

 the latter have the pairs of pores in the outer series closely set together, and the furrows 

 consequently indistinct. The widening of the avenues by poriferous sulci is most conspi- 

 cuously exhibited in Nucleolites sinuatus, and in this species and its allies the pores of the 

 ambulacra near the mouth have a tendency to fall into ranks of several pairs. 



The British Nucleolites known to me are the following ; — 



A. Species of the Oolitic Type. 



1 . Nucleolites clunicularis (including scutatus or depressus, and pyramidatus), as defined 

 in the preceding account of the species. 



2. Nucleolites dimidiatus (Sp.), Phillips, Geol. of Yorkshire, vol. i., pi. 3, f. 16. Coral- 

 line Oolite. 



N. ambitu ovato, antice rotundato, postice bilobato ; dorso convexo, apice centrali, vertice 

 subcentrali, postice tumido ; ambulacris anguste lanceolatis ; sulco anali prqfundo, ovato, 

 obtuso, superne abbreviato, lobis posterioribus tumidis ; ventre plus minusve concavo. 



This species rarely exceeds an inch in length, and varies greatly in the convexity of its 

 upper surface. The ovate anal sulcus, reaching about two-thirds of the distance betwc n 

 the posterior margin and the true summit, conspicuously distinguishes it from clunicularis, 

 with which it was confounded before being distinguished by Phillips. 



It occurs in both coral rag and cornbrash, but especially in the former. 



3. Nucleolites orbicularis (Sp.), Phillips, Geol. of Yorkshire, vol. i., pi. 7, fig. 3. 



N. ambitu orbiculari, lateribus tumidis, dorso convexo, apice vertice-que centrali, ambu- 

 lacris lanceolatis, ad apices distantibus ; sulco anali oblongo, obtuso, ad verticem approxi- 

 mato, lobis posterioribus obsoletis ; ventre concavo, ore subcentrali. 



Usual diameter about an inch and a quarter. Cornbrash of Wilts and Yorkshire. Coral 

 rag of Calne. The ocular plates are more apart, and the vertex consequently broader in 

 this than in any other British Nucleolite. 



4. Nucleolites Hugii, Agassiz, Echin. Suiss. vol. i., p. 35, t. 5. figs. 1-3. 



