﻿44 EVOLUTION IN THE PAST 



survivals of some ancient forms, that had initiated the Am- 

 monoid Order. 



All other ammonoids may be divided broadly into two 

 groups, — Goniatites and Ammonites. The Goniatites were 

 the earlier forms. Their shells w T ere of no great variety, and 

 were generally of plain outward appearance. The partition- 

 patterns — not, as a rule, externally visible — were in most 

 cases of extremely simple character. The Ammonite patterns 

 attained a wonderful degree of complexity. 

 . Many of the Devonian goniatites — ammonites had 

 not yet appeared — were probably active swimmers 

 (Gastrocampyli, Microcampyli). There is no doubt, however, 

 that crawling habits were in course of adoption. In some 

 forms (Clymenidce) the partition-patterns were characterised 

 by a slight elaboration. And this, no doubt, had resulted 

 from strains on the shell caused by crawling becoming more or 

 less habitual. 



Goniatites became very numerous in the course of the 

 Period, and increased in genera (Gephyroceras, Tornoceras, 

 Anarcestes, etc.) : but their shells continued of plain outward 

 appearance, being either smooth on the surface, or marked 

 only with delicate streaks. 

 univalved Gastropods for the most part were of the same character 



molluscs as those of the preceding Period. A few of these snail-forms, 

 however, had apparently developed a lung-chamber, and 

 become capable of breathing out of water (Rercynella). The 

 humble land-population was in prospect, therefore, of obtain- 

 ing more recruits. 

 bivalved Bivalved molluscs exhibited considerable development. 



molluscs Primitive oyster forms, belonging to the family of which the 

 saddle-oysters of our own day are members, were now abroad 

 (Limanornia). Other forms appear to have been well on the 

 way to becoming horse-mussels (Modiola) ; and burrowers 

 of the " razor-shell " type were also on the scene (PalcBosoleri). 

 Some forms had moved up from salt to brackish and fresh 

 water. Certain of the emigrants bore a close resemblance 

 to river-mussels (Archanadori). 

 polyzoans Polyzoan colonies, though suffering vast losses owing 

 to the rapidly increasing molluscs, and, possibly, fishes, 



