PCECILOMORPHUS. 115 



HildoceratidjE (continued). 

 Genus — Pcecilomorphus, 1 8. Buchnan. 



{Type — Pcecilomorphus ctcloides, d'Orbigny sp.) 



Definition. — Discoidal to subglobose, compressed or very thick, widely or very 

 narrowly umbilicate, carinate, furrowed ventrally ; whorls subquadrate ; ribs 

 sigmoidal, outer arc conspicuous and much longer than the inner, which is incon- 

 spicuous. Carina not elevated, solid, bordered by two furrows which are smooth 

 and as deeply marked on the test as on the core. Inner margin convex. Ter- 

 mination plain, with a short, rounded lateral, and a pointed ventral, process. 

 Suture-line : 2 — Siphonal lobe with two terminal points closely embracing the 

 siphuncle ; siphonal saddle with a small accessory lobe ; superior lateral lobe as 

 long as, or a little longer than, the siphonal, trifurcate; inferior lateral lobe 

 much shorter but similar ; one auxiliary lobe. The length of the chambers is 

 variable, apparently being closer together in proportion to the thickness of the 

 specimen. 3 



Remarks. —In thickness and the amount of involution the species of this genus 

 are very variable; but in ornamentation, shape of whorls, suture-line, furrowed 

 ventral area, &c, they show the same peculiar characters. From Hildoceras the 

 genus is separable by its possessing a convex, instead of concave, inner margin, a 

 narrower siphonal saddle, a thinner-stemmed trifurcate superior-lateral lobe, and a 

 wider superior-lateral saddle. From Ludwigia the absence of bifurcate ribs, the 

 absence of conspicuous ribs on the inner area, the presence of furrows on each side 

 of the carina, the convex inner area, and the absence of more than one auxiliary 

 lobe, easily distinguish it. From Lillia the sigmoidal ribs, the absence of knobs 

 or tubercles on the inner margin, the inconspicuous nature of the ribs on the inner 

 area, are the chief distinctions in addition to a narrower siphonal saddle. 



I have chosen Ammonites cycloides, d'Orbigny, for the type-form of this genus, 

 because it is a species with which workers in the Inferior Oolite of Dorset would 

 soon become familiar. There are two species belonging to this genus in the 

 Inferior Oolite ; and their ancestor is Ammonites Mercati, von Hauer, 4 which occurs 

 in the Upper Lias. Between this and the Sauzei-zone (the position of the first 



1 riuiKi'Aos, various ; ^opfv, form. 



2 For suture-lines of this genus see Plate A, figs. 31, 32, 33. 



3 I have not seen the suture-lines of a sufficient number of examples to be able to say if an exact 

 relationship exists between the thickness of the specimen and the closeness of the sutures. 



4 ' CephalopodenN.-O. Alpen. ; Denksch. Akad. Wissensch. mathem.-natur. CI.,' Bd. xi, pi. xxiii 

 figs. 4—10. 



