34 CIDARIS. 



(PI. I, fig. 4 a.) The plates composing the apical disc are absent in all the specimens 

 hitherto found. 



The spines (PI. I, fig. 4 d) are large ; the milled band below the neck is close to the 

 cup-like articulating cavity ; the neck increases slightly in thickness, and the nearly round 

 stem is covered with longitudinal rows of short, sharp prickles, which have their points 

 directed forwards ; the stem of the spine is slightly flattened. Fortunately one specimen 

 was found by Mr. Gibbs, of the Geological Survey, with the spine in situ attached to the 

 test, so that all doubt about the species to which it belongs is removed by this discovery. 

 The specimen with the spine attached is in the Geological Museum in Jermyn Street. 

 The secondary spines are short, and blunt pointed, but only a very few of these have been 

 found. Those belonging to the scrobicular circle are larger than those which armed the 

 small granules. 



Affinities and differences. — This urchin very much resembles an undescribed species 

 collected by M. De Loriere, from the Inferior Oolite of the department of the Sarthe. There 

 are some slight shades of difference between the French specimens and Cidaris Fowleri, 

 but the general resemblance between them is so very great, that we believe them to be 

 identical. The rock from which the French urchin was collected is referred to the " etage 

 Callovien ;" but it contains several species which hitherto have only been found by us in 

 the Inferior Oolite of England, as Holectypus gibberulus, Agassiz, Pygurus depressus, 

 Agassiz, and Clypeus Agassizii, Wright. From these facts we are disposed to think that 

 there must be a mistake about the true stratigraphical position of the formation from 

 whence these urchins have been collected. 



In the general structure of the test, and in its ornamentation, Cidaris Foivleri closely 

 resembles Cidaris Orbignyana, Agassiz,* from the Kimmeridge Clay of Havre. If the 

 palaeontologist had to decide the species from the anatomy of the test alone, he would have 

 great difficulty in distinguishing the one form from the other; but fortunately the 

 spines of both are now known, which settles the question as to the specific difference 

 existing between them. We have the test and spines of Cidaris Orbignyana before us, 

 but the reader will find fine figures of both in M. Desor's Synopsis, where it is admirably 

 figured for the first time, and to this work we beg to refer the reader. Compare, for 

 example, PI. Ill, fig. 13, spine of Cidaris Fowleri, with PI. VIII, figs. 7 — 9, the spine of 

 Cidaris Orbignyana. 



In the general form and structure of the test, Cidaris Foioleri resembles Cidaris fori- 

 gemma, but it differs from that well-known Coral Rag species in the flatness of its ambu- 

 lacral areas, in the greater breadth of the poriferous zones, its smaller primary tubercles, 

 the depth and character of the crenulations on the bosses, and, above all, in the structure 

 of its spines. Compare PI. I, fig. 4 d, with PI. II, fig. 2 a. 



* A beautiful figure of this fine species is published in M. Desor's ' Synopsis des Echinides Fossiles,' 

 pi. 1, fig. 3 ; pi. 8, figs. 7 — 9, contains the spines. 



