GRAMMOCERAS AALENSE. 193 



The next development (var. a) is seen in figs. 7, 8. This has become almost 

 smooth at an early age, and is more compressed ; it may be considered as the 

 senile form of the species. A variety (/3) which is more compressed than a, but 

 exhibits the united and irregular ribbing more forcibly, is depicted in figs. 4, 5. 

 Like a, it has a tendency to smoothness towards the latter part of the whorl. 

 Another variety (S) (figs. 9, 10) has what appear to be single ribs in the umbilicus, 

 and this feature causes a likeness to Gramm. lotJiaringicum (p. 199). 



A consideration of these, the principal forms of the species, will suggest the 

 transition from Gramm. fluitans to Gramm. leurum by the following arrangement : 

 fiuitans — aalense — aalense a — leurum. Each form exhibits at an earlier age the 

 senile characters of its predecessor accompanied by greater involution and com- 

 pression. 



Dr. Wright in his Monograph on Lias Ammonites placed several species 

 under the name Harpoceras aalense, but as I have not admitted them in the 

 synonymy, the following analysis may be useful to the reader of the present 

 Monograph : 



Harpoceras aalense, Wright (non Zieten). 



PI. Ixxv, fig. 8. Probably Lioceras ojpalinum. 



„ fig. 9. Gratmn. costulatum, (Zieten). 



,, fig. 10. — mactra (Dumortier). 

 PI. Ixxx, figs. 1, 2. Dumortieria Moorei (Lycett). 

 PL Ixxxii, figs. 1,2. — radiosa (Seebach). 



„ figs. 3, 4. — Levesquei (d'Orb.). 



The presence of joined ribs will separate Gramm. aalense from all the above 

 ' species ; and the difference in suture-line will distinguish it from most of them. 

 The first character may cause it to be confounded with Lioceras comptum (p. 53), 

 and with Ludwigia Marchisonse (p. 16). From the former it may be known by its 

 larger umbilicus and simpler suture-line — with fewer auxiliary lobes ; from the 

 latter by its ribbing being straighter on the lateral area, more projected ventrally, 

 and altogether more irregular. 



From species of Gi~ammoceras with joined ribs, Gramm. aalense is separated 

 as follows : — From Gramm. doerntense by greater compression and involution ; from 

 Gramm. dispanstim by less compression, larger umbilicus, coarse irregular ribbing, 

 and a solid carina ; from Gramm. suhco7}iptum by coarser ribbing not so fascicled. 



In this country the horizon which is characterised by Grammoceras aalense is 

 very narrowly defined ; for I have not found the species outside the limits of the 

 ilfooret-beds ; and I consider it is indicative of that series. Dr. Haug,^ however, 



1 Op. cit., p. 667. 



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