GRAMMOCERAS DOERNTENSE. 



183 



work. He devotes the greater part of plate viii to a series of drawings of this 

 species in various stages. His fig. 5 seems to me to indicate a stage intermediate 

 between this species and Gramm. toarcense with joined ribs. 



Just the opposite to this is the variety of which an adult is figured (Plate XXIX, 

 figs. 8, 10) and — to show that it is not merely the adult stage of the figures above 

 it — a smaller specimen has been depicted (Plate XXXIII, figs. 11, 12). This variety 

 is noticeable for the absence of the irregular and joined ribs so characteristic of 

 Gramm. doerntense, and this character, together with the finer ribbing, is well 

 brought out in the smaller specimen. It appears to me that this variety is a 

 further mutation of Gramm. doerntense proper — a mutation in which the joined 

 ribs have disappeared, and the ribs themselves have become finer generally ; and 

 in consequence of these characters this form converges towards Gram/m. Orhignyi. 

 It might reasonably be supposed that Gramm. Orhigmji was only the further deve- 

 lopment of this form, and that the proper order of descent might be thus 

 expressed : 



toarcense, 



variety, 



joined ribs 



in adult. 



C Intermediate form," 

 ■< joined ribs not 

 (. continued to end. 



doerntense, 



joined riba 



in youth only. 



' doerntense, 



variety, 

 _no joined ribs._ 



Orhignyi, 



more compression, 



reversion to some 



obscure joined ribs. 



Now, although the first four stages are, to my mind, correct enough, I am not 

 inclined to think that the last is descended in this manner. The reason is that 

 Orbignyi has a slightly larger umbilicus than doerntense variety, and it also seems 

 to me much more probable that it is descended directly from Gramm. striatulum ; 

 because it is what we might call a normal mutation of that species (p. 185). Still 

 the subject requires further investigation. 



The horizon of this very scarce species is in the Dlspansum-hed (Jurense-zone), 

 so that it really occurs one stage later than the time when Gramm. toarcense was 

 dominant. Its development with regard to that species agrees exactly with its 

 position. 



I have found this species at Coaley Wood and Bnckholt Wood in Gloucester- 

 shire, in this horizon. I have one characteristic little specimen (figs. 6, 7), labelled 

 South Petherton, Somerset. Its horizon cannot be stated any nearer than in the 

 so-called " Upper Lias " of that district, that is, above the Marlstone and below 

 the Yeovil Sands ; but I have no doubt it would be found to correspond exactly, 

 from a palseontological point of view, with the horizon of the Gloucestershire 

 specimens. It should be noticed that these occur in the second stage above the 

 Cotteswold Sands, affording evidence of the distinctly different position of the two 

 deposits of sands in a palgeontological sense. 



