AMMONITES.

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Many species of Ammonites are distinguished by a large lateral process formed on eachside 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 A. rostratus, which is commonin 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 very peculiar struc-ture, which have received various names, and still more discordant explanations, fromdifferent authors. Parkinson called them Trigonellites on Meyer Aptgchus, DeslongchampsMunsteria. They have been thought by many writers to be external bivalve shells of theLamellibranchiate order; or internal shells of Cephalalopods; Riippell thought some ofthem 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. Butthe opinion which seems to have most probability in its favour is that the Trigonellites werethe opercula of Ammonites, which view has been ably supported by Yoltz; 1 the principalreasons for this opinion are derived from the structure of the plates being analogous tothat of many opercula, and their having been often found within the open chamber ofAmmonites, to the mouths of which their form and size nearly correspond. Each plate istriangular, with one straight edge, which laps over or under the corresponding edge of itsfellow plate. The lines of growth are always seen on the concave side of the plates, show-ing that to have been the external surface. Yoltz divides the Aptychi (or Trigonellites)into three sections: 1st, the cornel , consisting of a single (?) horny plate, with a flexiblefold down the middle; 2d, the imbricati, which have a pair of plates with an externalhorny epidermis, covered internally, that is on the convex side, with calcareous layers laidover 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 theChalk; those which have been found will be described in connection with the Ammonitesto which they are supposed to belong.

In the first volume of the 'Journal of the Geological Society, 5 p. 232, Mr. Stricklandhas described some supposed opercula of Ammonites, consisting of one semi-oval calcareousplate, without any suture, which have been found in the Lias: no similar form has yetbeen found in the Chalk.

1. Ammonites comtlanatus, Mantell . Plate YII, figs. 1—3.

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

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

— lakgilliertiantjs, D'Orbigny. Paleont . Franc. Terr. Cret., pi. xcv.

A. testa discoided, compressissima, Iceviuscula, tenuiter striata; striis numerosissimisjlexuosis , umbilicum, versus obsoletis, deinde distinctis et ad margines dorsi sub-tuberculatis;

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