4 02 SCIENCE PROGRESS 



displayed unrolled forms so early as the Callovian formation 

 of the Jurassic ; they are probably derived from the family of 

 Rciucckida.', for their sutures are similar to those of Rcineckia 

 (fig. 13), but they have been reduced to the primitive formula 

 of six lobes. (Ecopty chins (fig. 14) already shows a tendency 

 to being pulled out ; in Spiroceras (fig. 1 5) the spiral has become 

 quite an open one ; and in Baculina (fig. 16) it has become 

 completely straightened out into the shape of a needle. 



Other groups of the Pachycampyli show a very similar 

 parallelism. For instance, in the Acanthoceratidce we can pass 

 from the normal Lyticoceras (fig. 17) to the open and pulled out 

 spiral of Helicoceras (fig. 18), and the more irregular, half-closed, 

 half-open Hctcroceras (fig. 19); finally we reach the open bow 

 of Toxoceras (fig. 20), and the straightened double hook of 

 Tomeutoceras (Hamites) (fig. 21). It will be noticed that all 

 these forms, as well as those of the next group, are of 

 Cretaceous age. 



The group of the Cosmoceratidcc, the members of which are 

 so highly ornamented with ribs and tubercles, show even more 

 aberrant variations of the spiral curve. Here again we can 

 trace a transition from the normal Donvilleiceras (fig. 22) to the 

 open spiral of Crioceras (fig. 23), and the hook-like Anisoceras 

 (fig. 24) and Ancyloccras (fig. 25); the last-named may be com- 

 pared with the more closely coiled Macroscaphitcs (fig. 7). The 

 adult Anisoceras combines the open spiral of Helicoceras with 

 the bow of Toxoceras, and in its old age terminates in a crook or 

 retroversal bend. This group does not contain any straight 

 forms, but the turreted spiral is well exemplified by Turrilites 

 (fig. 26), which was already foreshadowed in the Trias by 

 Cochloceras (fig. 4) ; and in Bostrychoceras (fig. 27) the turret 

 has become in part an open spiral. Empcroceras shows a 

 remarkable instability, varying according to age ; the young 

 remain in a Hamitean stage for a prolonged period, and then 

 suddenly develop into a form like Helicoceras or even Turrilites. 



A key to the cause of this unrolling of the spiral in these 

 families of Ammonites, prior to their extinction, is perhaps 

 furnished by the fact that the number of lobes in these aberrant 

 species has undergone reduction, and reverts to the primitive 

 number of six. The evolution of more or less uncoiled or 

 straightened forms may, therefore, be due to a struggle, so to 

 speak, to return to primitive simplicity both of outward form 



