. Cephalopoda. Cephalopoda. 113 the Sepioidea (q. v.), which require a more massive envelope around the phragmocone. In addition, such an indifferent state of the rostrum is present in a number of fossil Decapoda (Belemnoteuthis, Diploconus) including the most ancient known form (Phragmoteuthis from the Triassic), which in any case has to be considered as the starting point for the Dibranchiata. (Ill) (iii;. FIGURE 38. Typical rostra of belemnites of the group Clavirostridae (after E.Stolley, 1911, from O.Abel, 1916): A — Acroteuthis; B — Oxy teu this; C —Hibolites. The juvenile rostrum (wrong
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. Cephalopoda. Cephalopoda. 113 the Sepioidea (q. v.), which require a more massive envelope around the phragmocone. In addition, such an indifferent state of the rostrum is present in a number of fossil Decapoda (Belemnoteuthis, Diploconus) including the most ancient known form (Phragmoteuthis from the Triassic), which in any case has to be considered as the starting point for the Dibranchiata. (Ill) (iii;. FIGURE 38. Typical rostra of belemnites of the group Clavirostridae (after E.Stolley, 1911, from O.Abel, 1916): A — Acroteuthis; B — Oxy teu this; C —Hibolites. The juvenile rostrum (wrongly named "embryonic rostrum") is shown in black. Its bowl-shaped base sur- rounds the embryonic chamber of the phragmocone. The rostrum remains always club-shaped, whatever its later modifica- tion. This juvenile rostrum does not rep- resent the initial part of the rostrum which forms in fact the axial line which is the growth center of the juvenile rostrum (cf. p. 134). FIGURE 39. Typical example of a "conirostrid" (after F. A. Quenstedt from Abel, 1916). Longitudinal section through the upper part of the rostrum of Belemnites gigan- teus Schloth (Mucroteuthis Abel). The phragmocone is intact and delimited by the conotheca. The growth layers are actually paper- thin; the figure combines several of them (semi-diagrammatic). An average form like the conirostrid belemnites (Figure 39) may be considered as the ancestral form of the rostrum of Decapoda. At any rate, both the typical rostrum and its sheath are concentrically stratified and consist of a strongly calcified shell mass. The proostracum of Protodecapus (Figure 40, p. 112) is distinctly differentiated into middle and lateral plates (Figure 41). The middle plate is pointed triangular. It extends in an acute angle from the rounded anterior end to the end of the cone, its posterior part belonging to the conotheca. 104. Please note that these images are extracted from scanned page images that may hav