Author Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1897 Excerpt:...is seen the pear-shaped siphonal caecum or knob at the beginning of the siphuncle (Pl. XVI, fig. 4). This is surely an embryonic feature, being present before the development of the first septum, and probably represents the shrunken, horny covering of the nautiloid embryos, while the calcareous shell of the protoconch is an ammonoid character pushed back by acceleration of development until it occurs simultaneously with nautiloid characters. No trace of a prosiphon could be seen on any specimen, although some were nearly as transparent as glass and would surely show this organ in transmitted light. Ananepionic. With the appearance of the first septum (Pl. XVIII, fig. 1) the animal becomes not merely a cephalopod, but a chambered nautilian shell, and this is regarded as the beginning of the larval period proper. As shown on Pl. XVI, figs. 3 and 4, this suture consists of a rather narrow, rounded, siphonal saddle, flanked by a narrow, lateral lobe and broader, lateral saddle. This stage corresponds to some Silurian nautiloid, and while constant in all ammonoids is of equally short duration in all, lasting for only one septum. Metanepionic. At the second septum the shell enters the second larval stage, is no longer a nautiloid, but with the development of a siphonal lobe becomes an ammonoid. As Hyatt1 has shown, this stage has in most ammonites, especially the older forms, an undivided abdominal lobe like that of the Nautilinidae of the Lower Devonian; but in many later and more highly accelerated genera the stages corresponding to the older goniatites are omitted. Thus in Lytoceras alamedense (Pl. XVIII, fig. 2, and Pl. XVI, figs. 4 and 5) at the second septum the abdominal lobe is developed, but it is already divided by a siphonal saddle; this Hyatt thinks c...