﻿142 MR. S. S. RUCKMAN ON CERTAIN [Feb. I905, 



9. On Certain Genera and Species of Lytoceratid^:. 

 By S. S. Buckman, F.G.S. (Bead December 21st, 1904.) 



[Plates XV & XVI.] 



I. Certain Lytoceratid^; erom the Northampton 

 Sands (Aalenian). 



Some time ago Mr. Beeby Thompson, F.G.S. , sent to me for identi- 

 fication certain species of Lytoceratidae from the Northampton Sands. 

 As one is new and remarkable for its homceomorphy to Phylloceras, 

 and the others are interesting, I desire to offer the following notes 

 to the Geological Society. 



In order to classify the series of Toarcian-Aalenian Lytocera- 

 tidae, to which these species belong (jurense and allied groups), it 

 is necessary to note that there is ontogenetic evidence that they 

 have passed through a more or less definite sequence of phylo- 

 genetic stages. In regard to ornament, there is a sequence of 

 development — first some elaboration, but, later, simplification from 

 a costate (or corrugate) to a completely-smooth stage. In regard 

 to whorl-shape, there is a constant tendency to pass, more or less 

 in pace with the decline in ornament, from the Lytoceratan to the 

 Phylloceratan form — that is, from an evolute to an involute whorl, 

 the umbilicus constantly contracting. Conjointly with such change 

 there is a further tendency, to pass from stout to constantly more 

 compressed whorls. 



Now, Lytoceras originally took its name from the evolute 

 character of its whorls (\vt6s) ; Phylloceras from the character 

 of its lobe-line (^vXXov). But the whorl-character of Phylloceras 

 is the opposite of Xvtos : it is involute ; and that is what is 

 implied by saying ' phylloceratan whorl-shape.' The main difference 

 between the Lytoceratidae and the Phylloceratidae may be ex- 

 pressed in this way : the first have the whorl-shape more primitive 

 and the lobe-line more advanced (less of the phylloid, more of the 

 elaborately-denticulate pattern) ; the second have the whorl-shape 

 more advanced and the lobe-line more primitive (more phylloid). 

 Consequently, when the Lytoceratidae advance in regard to whorl- 

 shape to the Phylloceratan style, they become characterized by 

 an advanced whorl-shape and an advanced lobe-line. The whorl- 

 shape being, then, similar to that of the Phylloceratidae, there 

 is a certain external homceomorphy ; but the more advanced 

 lobe-line remains a feature of distinction, showing that a given 

 species belongs to the Lytoceratan stock, however much it may 

 simulate the Phylloceratan shape. 



It may be desirable to explain that, in speaking of the Xvros- 

 character as primitive, I refer only to that portion of the Cephalopod- 



