252 SYNOPTIC COLLECTION. 



it makes a siphuncle lobe (No. 601, specimen on the 

 right), which later in life in some species becomes divided, 

 giving rise to the siphuncle saddle. 



The Goniatites cease to exist in the Carboniferous age 

 and the Ceratites appear. The specimen of Ceratites 

 (No. 602, C. nodosus De H.), does not show the younger 

 whorls but the sutures of the last whorl are finely seen. 

 The number of saddles and lobes has increased but still 

 can be easily counted. The bottom of the lobes is ser- 

 rated, which may be seen in several places in the speci- 

 men. 



The more primitive Ammonitidae are represented by 

 Deroceras (= Ammonites) planicosta Hyatt. Here the pro- 

 toconch is large and globular (PI. 603, fig. i). In this 

 figure the shell is seen on the right with a few septa, and 

 on the left it is broken exposing the cast of the proto- 

 conch. Fig. 2 is a section of the same. The contraction 

 of the siphuncle is well seen. Fig. 3 is a section through 

 the shell with the protoconch broken off. The coecum 

 formed by the first septum is finely shown ; three other 

 septa are drawn. An organic deposit fills the siphuncle. 

 Fig. 4 is a section through the shell of a related genus, 

 Plenroceras spinatum Branco, showing the protoconch in 

 the center, the coecum and siphuncle, the first concave 

 Nautiloid septum, and the remaining convex Atnmonoidal 

 septa. It is interesting to know that the septa of the first 

 formed chambers are simply curved, while in the adoles- 

 cent stage they resemble those of Goniatites, having few 

 lobes and saddles (see fig. 5). In the whorls of the adult 

 they are always folded (see outer whorl in fig. 5). 



In this adult shell the ventral sinus of the aperture, so 

 conspicuous in the Nautiloids, has disappeared and the 

 lines of growth are continued straight across the ventral 

 portion. This being the case we must infer that the 

 organ which caused the sinus has disappeared. Hyatt 1 



1 Genesis of the Arietidae, Smithsonian Contrib. to Knowledge, 

 XXVI, 1889. 



