A MONOGRAPH 



OF 



BRITISH FOSSIL TRIGONLE. 



GENERAL OBSERVATIONS AND SYSTEMATIC ARRANGEMENTS. 



Throughout the great Mesozoic epoch there are, perhaps, no testaceous forms, with the 

 exception of the Ammonites, of importance superior to the Trigonise, or which demand 

 from the Palseontologist more careful discrimination and more extensive acquaintance 

 with their various aspects, to enable him to overcome the difficulties that meet him when 

 he attempts their certain determination. The importance of the genus is based upon its 

 great stratigraphical range, its world-wide occurrence, its great diversity of aspects, 

 locally also by its individual numbers. Its prominence and distinctness as a genus 

 immediately arrests the attention of the observer, and, notwithstanding the diversity of its 

 aspects, a Trigonia is a form recognised without difficulty even by a tyro. The generic 

 characters are so well known that to give a minute description of them would amount to 

 a mere useless repetition of that which has been so fully accomplished by Agassiz and by 

 other Pala3ontologists. I prefer, therefore, to allude to them with the greatest possible 

 conciseness, and will adopt the terse and brief definition given by my late lamented 

 friend, Dr. S. P. Woodward, in his well-known ' Manual of the Mollusca :' — "Shell 

 thick, tuberculated, or ornamented with radiating or concentric ribs j posterior side 

 angular, ligament small and prominent ; hinge-teeth 2 — 3, diverging, transversely 

 striated ; centre tooth of the left valve divided ; pedal impressions in front of the posterior 

 adductor, and one in the umbo of the left valve ; anterior adductor impression close to 

 the umbo. Shell almost entirely nacreous, and usually wanting or metamorphic in 

 limestone strata." 



Moulds which represent the internal cavity of the shell may readily be obtained 

 whenever the investing matrix is in its structure more coherent or compact than the 

 fossilized test, and it usually happens that a rock or matrix or great hardness is associated 

 with a test more than usually friable. The value of these internal moulds to the Palseon- 

 tologist who wishes to discriminate species is, however, only limited ; it rarely happens 



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