24 NORTH AMERICAN INDEX FOSSILS. 



i486), or by a row of tubercles (Fig. 1495). A keel, solid or 

 hollow, may further modify the venter in both compressed forms 

 (Figs. 1409, d; 1442) and those with broad venter (Fig. 1503). 

 This keel in some cases is bounded by depressed grooves (Fig. 

 1409, g) or distinct channels (Fig. 1507). It is continuous in 

 most forms (Fig. 15 10), but in some cases becomes broken into 

 elongated nodes (Fig. 1509, c). In all cases the keel is a mark 

 of specialization. 



Ornamentation of the Surface. — In many forms the surface is 

 perfectly smooth, though longitudinal striations and transverse 

 annulatiofis are characteristic of some of the earliest forms (Figs. 

 1241, 1258). In coiled forms the longitudinal markings are the 

 spirals, and the transverse the ribs or costce. Where costse and 

 spirals intersect, a cancellated structure is produced, when both are 

 fine and uniform. When strong spirals cross the ribs, or where an 

 angulation is crossed by the ribs, indefinite swellings, or nodes, 

 definite rounded tubercles, or even spines, may be produced (see 

 Figs. 1470, 1469 and 1458, respectively). The nodes may be elon- 

 gate, as the result of disruption of a strong spiral (Fig. 1507). 

 The ribs may be coarse or fine and sharp, simple or dividing, 

 regularly or irregularly. Division may be by forking {bifurcation, 

 trifur cation, etc.) : the regular division into two or three equal, 

 and equally diverging branches (Fig. 1448), or by branching — 

 a lateral branch being given off the main continuous one. Fre- 

 quently a node or tubercle is formed at the point of furcation. 

 Increase in the number of costae is also effected by intercalation, 

 or the appearance of a new secondary rib between the older ones. 

 These secondary ribs often do not reach the umbilical margin 

 (Fig. 1474). Furcation and intercalation may occur in the same 

 shell. Various other surface features occur, such as angulations, 

 channels which may interrupt the costae, frilled growth lines, etc. 



Special Features of the Dibranchiate Shell. — In the Dibran- 

 chiata, the camerated shell, when preserved, is known as the 

 phragmocone. It is straight, curved, or coiled (Spirula), but in 

 a number of specialized types it is wanting altogether. In the 

 Belemnoidea, the delicate shell or conotheca of the phragmocone 

 is prolonged forward into a delicate corneo-calcareous plate, the 

 proostracum. Posteriorly it is enveloped by a calcareous finger- 

 or cigar-like sheath or guard {rostrum) (Figs. 1511-1515). The 



