GEOLOGY. 213 



TRIGONELLITES LATUS. 



( Woodward's Recent and Fossil Shells, p. 80. Von Bucli, on 

 Aptychus : Geological Journal, Vol. vii, p. 32. ) 



[ Read at the Museum, Corfe Castle, Feb. 20th, 1860. ] 



The Trigonellites are among the few well preserved Fossils 

 of the Kimmcridge Clays of Purbeck: they seem to have escaped 

 the decomposition to which other organic matter was subject 

 With the exception of Saurians and Vertebrate Fish, impressions 

 only remain of the various forms of life which inhabited the seas 

 during the deposition of these beds . The Kimmeridge Clays, 

 which sink into the sea at Gad Cliff, do not again appear until 

 after an interval of eight miles, at Ringstead Bay, where they 

 form the transition from the clay into the Oxford Oolite. 



The fossils are well preserved, and do not crumble away as 

 those at Kimmeridge. The bone- like structure of the Trigonel- 

 lites may possibly be the cause of their preservation, for Saurian 

 and Fish bones are plentifully distributed, uninjured by the 

 decomposing property of the shales. 



The form of the Trigonellites as their name represents, is tri- 

 angular, two of its angles are pointed, one is gibbous; the sides 

 abutting the latter have sharpened and bevelled edges, the other 

 is truncated. Distinct lines of growth appear on the concave 

 surface, the convex is spotted and has the appearance of shagreen. 

 Naturalists are not unanimous as to their position in the organic 

 world. Agassiz and D'Orbigny rank them either among the 

 bivalves or the cirripides, but after a slight examination the 

 absence of an adductor muscle, palial sinus, and umbo, \vill 

 show the erroneousness of this view. With greater probability 

 they might be assumed to be the operculum of a Cephalopod which 

 nature supplied with a peristomal defence to overcome difficulties 

 which some members of this family had to encounter. Tliey 

 must be viewed in pairs, and acting as folding doors to preserve 

 the mollusc from haiiu cither when drawn within, or protruded 



