Vol. 69.] CERTAIN T7PPEK JURASSIC STRATA OF ENGLAND. 429 



Rasenia, gen. nov. 



Genotype, Ammonites ci/modoce (VOrbigny, 'Pal. Franchise : Terr. Jurass.' vol. i 

 (1850) pi. ccii, figs. 1 & 2 \non rigs. 3 & 4), & pi. cciii, fig. 1. 



Another characteristic species is Ammonites ur'alensis d'Orbigny, in Murclii- 

 ■ son, de Vemeuil, & Keyserling, ' Geologie de la Russie d'Europe ' vol. ii (1845) 

 pi. xxxii, figs. 6 & 7 [non figs. 8 & 9). 



Rasenia comprises many of the so-called Olcostephani of the 

 Kimmeridgian. The above-mentioned species belong to two distinct 

 groups of Rasenia;, of which the first becomes smooth with age, 

 while the second acquires strong, undivided ribs. Rasenia is a 

 genus rich in species, which have their chief development in the 

 Kinimeridge of Northern France, England, and the interior of 

 Russia, but occur also as frequent accessory faunal constituents in 

 Southern Germany, Switzerland, the middle and south of France, 

 • and the Alpine Jura. 



The name is derived from Market Piasen (Lincolnshire). 



jPerisfhinctes eastlecotxensis, sp. nov. (Pis. XLI & XLII.) 



Ammonites pectinatus of English authors (in part). 



Measurements. 1 



Diameter 



Breadth of 



Thickness of 



Width oj 



in 



whorl. 



whorl. 



umbilicus. 



■millimetres. 



(Percentage 



(Percentage 



(Percentage 





of dia.ni.) 



of diaiu.) 



of rliani.) 



170 



37 



36 



37 



145 



36 



37 



37 



Description. — This form possesses an oval whorl-section, even 

 in youth; the greatest thickness is close to the umbilicus. The 

 umbilical edge is rounded, the umbilical slope (inner margin) steep. 

 The inner whorls overlap one another by about a half; in later 

 whorls the overlap is rather less. 



The ribbing is very fine and relatively weak, while the number 

 of primary ribs is very great; at a diameter of 110 millimetres 

 there are about 130 in a single whorl. At a diameter of 145 mm. 

 the main ribs become gradually more widely spaced and stronger, 

 while the number of external ribs, on the other hand, remains 

 approximately the same. The ribs rise obliquely backwards 

 from the umbilicus, and soon take a forward bend, running 

 almost radially across the lateral area. The ribs divide in an 

 irregularly fasciculate (virgatite) manner, and at all stages of 

 growth pass over the periphery without any break. The first 

 furcation of ribs occurs about the middle of the lateral area. 



The inner whorls show, up to a diameter of about 85 mm., 

 flattened, narrow constrictions almost parallel with the ribs. 



The lobes and saddles are very finely divided. The deep external 

 and first lateral lobes lie on the same radius ; both are relatively 

 broad. The second lateral lobe extends to somewhat more than 



1 For tbe method of giving proportions, see S. S. Buckman, ' Yorkshire 

 'Type Ammonites' pt. 9 (1913) p. viii. 



