THE CEPHALOPODA 167 



The ornamentation of ammonoids falls under a few 

 definite heads. Primitive genera are smooth, and so are 

 many higher forms in early youth. The smooth stage is 

 generally followed by a capillate stage, in which there is a 

 pattern of fine raised lines on the surface ; and this by 

 subcostate and costate stages, in which the much more 

 prominent features called ribs (pilcz, costa) are developed : 

 these are really corrugations of the shell, and usually 

 appear as strongly on the cast as on the exterior ; when 

 they do not, it is because an internal partition or septum 

 is formed under each rib, as in the planulate ammonites 

 of the Upper Lias. A still later stage is the tuberculate 

 or spinous, when portions of the ribs stand out promi- 

 nently either with rounded or pointed ends. These 

 stages follow one another in the given order (anagenesis 

 of ornament), though any one of the intermediate stages 

 may be skipped in the hurrying-on of development. 

 After the last stage, catagenesis may often take place, 

 and the shell which is tuberculate in youth may become 

 costate or even smooth in old age. 



The ribs, or capillae, run in a straight or curved direc- 

 tion from the umbilicus to the periphery ; their course is 



FIG. 49. WHORL-SHAPE IN AMMONOIDEA (continued). 



umbilicate, lenticular oxycone, whorl-section cordate, d, Amaltheus, 

 angust-umbilicate, discoidal oxycone, whorl-section sagittate, e, Dia- 

 gram based on Aspidoceras to explain areas of whorl-surface, f, Erym- 

 noceras, coronate, whorl-section greatly depressed (periphery and 

 umbilical area practically meet, excluding any lateral area), g, Peri- 

 spkinctes, section showing early whorls of depressed crescentic shape, 

 changing later to compressed and planulate. h, Quenstedtoceras , 

 section showing early compressed whorls, changing later to broad and 

 somewhat depressed, i, Cadoceras, a cadicone, greatly depressed 

 whorls, deep conical umbilicus (indicated by dotted lines), a, b, c, d, 

 f, i, After d'Orbigny ; g, after Dumortier and Fontannes ; h, after 

 R. Douvilte. 



