AMMONITES. 19 



Many species of Ammonites are distinguished by a large lateral process formed on each 

 side of the mouth, either when the animal is full-grown or at intervals during its growth. 

 Such forms are mostly found in the Oolitic Formations, but J. rostratus, which is common 

 in the Upper Green Sand, affords a good example of such a mode of growth. 



In many of the secondary strata are found pairs of shelly plates of ver}^ peculiar struc- 

 ture, which have received various names, and still more discordant explanations, from 

 different authors. Parkinson called them Trigonellites^on Meyer Jpti/chus, Deslongchamps 

 Miinsteria. They have been thought by many writers to be external bivalve shells of the 

 Lamellibranchiate order; or internal shells of Cephalalopods ; Riippell thought some of 

 them scales of Reptiles ; some have been regarded as palatal teeth of Fishes ; and M. 

 D'Orbigny has lately considered them the side plates of pedunculated Cirrhipedes. But 

 the opinion which seems to have most probability in its favour is that the Trigonellites were 

 the opercula of Ammonites, which view has been ably supported by Voltz;^ the principal 

 reasons for this opinion are derived from the structure of the plates being analogous to 

 that of many opercula, and their having been often found within the open chamber of 

 Ammonites, to the mouths of which their form and size nearly correspond. Each plate is 

 triangular, with one straight edge, which laps over or under the corresponding edge of its 

 fellow plate. The lines of growth are always seen on the concave side of the plates, show- 

 ing that to have been the external surface. Voltz divides the Aptychi (or Trigonellites) 

 into three sections : 1st, the cornei, consisting of a single (?) horny plate, with a flexible 

 fold down the middle; 2d, the imhricati, which have a pair of plates with an external 

 horny epidermis, covered internally, that is on the convex side, with calcareous layers laid 

 over one another like tiles; 3d, the cellulosi, which have a pair of plates with a thick, 

 cellular, calcareous layer on the inner or convex surface. The Trigonellites are rare in the 

 Chalk ; those which have been found will be described in connection with the Ammonites 

 to which they are supposed to belong. 



In the first volume of the 'Journal of the Geological Society,' p. 232, Mr. Strickland 

 has described some supposed opercula of Ammonites, consisting of one semi-oval calcareous 

 plate, without any suture, which have been found in the Lias : no similar form has yet 

 been found in the Chalk. 



1. Ammonites complanatus, Mantell. Plate VII, figs. 1 — 3. 



Ammonites complanatus, Mantell. Fossils of the South Downs, p. 118. 



— — Sowerby. Min. Conch., t. 569, fig. 1. 



— LARGiLLiERTiANUS, D'Orbigny. Paleont. Fran9. Terr. Cret., pi. xcv. 



A. testa discoided, compressissimd, Iceviusculd, tenuiter striata ; striis numerosissimis 

 Jlexuosis, iDiibilicum versus obsoletis, deinde distinctis et ad margines dorsi sub-tuberculatis ; 



1 Neues Jahrbuch, 1837, pp. 304 and 432. 



