1883.] 255 [Hyatt. 



The forms of the earliest fauna agree in their general aspect 

 owing to the proximity of the septa, but they do not agree in 

 structure, or in their embryos. 



The embryo in the Nautiloidea is a shrivelled protoconch, 

 which may have been rounded at first but must have become 

 shrunken and shrivelled after the animal passed out of it and into 

 the apex ; it does not contain the siphonal coecum, and when 

 broken away left a narrow cicatrix on the apex, the opening 

 closed by a layer of shell. The umbilical perforation is an 

 opening through the centre of the whorls of even the most 

 completely coiled modern Nautilus, the hereditary mark of its 

 uncoiled ancestry. 



The siphon may be near the venter, but the funnels rarely, 

 if ever, break the continuity of the suture. 1 The funnels of the 

 siphon are simple posterior prolongations of the septa. The 

 sutures are entire, they never have marginal lobes and saddles 

 or more than two lateral lobes ; the ventral lobe is usually undi- 

 vided or simply Y-shaped when it occurs, 2 the dorsal lobe is, also, 

 usually undivided, but may be divided in rare cases by saddles, 

 the annular lobe when it occurs is undivided. 3 



The siphon is variable in position, but the larger number of 

 ancient genera have the siphon ventral or near the convex side. 

 It shifts in nearly all the series to near the centre, or dorsal 

 side of the centre in the higher and often later occurring nautilian 

 forms. 



The Ammonoidea have a globose protoconch, containing the 

 coecum of the siphon, and when broken away it leaves the apex 

 open. There is no umbilical perforation, except in the lowest and 

 earliest of the Goniatitinae, the Nautilinidae. Some of this 

 family have straight apices or young, others among them have 

 arcuate and gyroceran stages, without the orthoceran, while the 

 most closely coiled species in the adult are also close coiled in the 

 larva and do not have arcuate and gyroceran stages. The higher 

 Nautilinidae and all the succeeding genera have close coiled 

 whorls in the earliest stages, with exceedingly rare exceptions, 



1 Except, perhaps, in the Frochoanoids ? 



2 It may become divided by a broad saddle, the median saddle in rare cases. 



8 Except in Trematodiscus and similar forms, where a median saddle is developed. 



