FROM THE LO^^'ER GREENSAND 89 



The ambulacra! areas are wide (fig. 3 a, b) and have two rows of tubercles placed on 

 the margin of the area ; these are small, uniform in structure, set closely together, and 

 gradually diminish from the equator to both poles ; a band of granulations down the 

 middle of the area divides the two series from each other : the poriferous zones are narrow 

 and straight (fig. 3 b) ; the pores are round and simple, and arranged in a single file 

 throughout the zones (fig. 3 c). 



The inter-am]julacral areas are occupied at the ambitus by four rows of tubercles ; the 

 inner rows extend from the mouth to the disc, and the outer rows diminish in size on 

 the upper surface and disappear before reaching the disc ; the tubercles forming the inner 

 row are about the size of those in the ambulacra ; those of the outer row are sensibly 

 smaller (fig. 3 b). The miliary zone is large, and slightly depressed near the summit ; it 

 is filled with numerous granules of unequal sizes, some of which are mammUlated and 

 perforated ; the granules are disposed in circles around the areolae, and fill the entire area 

 of the zones with a beautiful oniamentation ; the examples from Farringdon have lost 

 much of this character from the process of fossUization in those gravel beds. 



The base of the test is flat, and presents a highly tubercular surface (fig. 3 a), the foiu* 

 rows of tubercles in the inter-ambulacral areas being all distinctly developed in this 

 region. The mouth-opening, one half the diameter of the test, is proportionally large : 

 the peristome is deeply notched into the lobes, the ambulacral portions being one half larger 

 than those of the inter-ambulacral arches. In fig. 3 c, I have given a section of the 

 base, magnified four diameters, showing the relation of all these parts to each other. 



Affinities and Differences. — ^This Urchin presents many varieties of form, which have 

 been described by difierent authors as so many distinct species, an error that has been 

 now corrected, as shown in the table of synonyms. It resembles P. Bourgneti, Ag., 

 found with it in the same Xeocomian beds, but is distinguished from that species 

 in having the primary tubercles less developed, more closely set together, and more 

 homogeneous ; and in the secondary or outer series of tubercles being larger and more 

 reffularlv arranged ; thev are, however, nearlv allied forms of one tvpe of structure. 



Localifi/ and Strafi^raphical Poniion. — This Trchin in England has hitherto been 

 found onlv in the sands and gravels near Farringdon, where it is extremely rare. It was 

 collected from these beds by the late Mr. D. Sharpe, and I obtained one specimen in the 

 same locality. On the continent of Europe it is one of the most characteristic fossils of the 

 "Terrain Xeocomien," and is found principally in the middle beds of that formation. 

 'hi. Cotteau records the following localities in France where it has been collected : — Billecul, 

 Mieges, et I'ermitage de Censeau, Nozeroy (Jura) ; Morteau, Hautepierre (Doubs) ; Ger- 

 migney (Haute-Saone) ; Vassy, Bettancourt, (Haute-Marne'i ; Thiefi'rain, Vandoeu\Te, 

 Marolles (Aube) ; Cheney, Flogny, Moneteau, Auxerre, Gy-rEveque, Leugny, Fontenoy, 

 Saints, Pereuse (Yonne) ; in all these locahties it is collected in abundance from the 

 Ikliddle Xeocomian ; and at Le Rimet (Isere), Villefargeau, Perrigny (Yonne), it is very 

 rare in the Upper Neocomian. In Switzerland it is found near Locle in the Lower 



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