MUSEUM OF COMPARATIVE ZOOLOGY. 63 



of the ovisac, and that the outer layer of the ovisac corresponded with 

 both the inner and outer layers of the first whorl. Both of the latter 

 are continuous with the outer layer of the ovishell, in the same sense 

 that they are subsequently continuous with each other whenever the 

 growth is interrupted, and an imbricated suture or break is formed. 



The shell of the young and mature whorls is composed of an inner 

 and an outer layer, the latter being divided into two portions, the 

 external or colored stratum, and the internal or white stratum. In 

 Nautilus both the layers extend entirely around the shell in the young, 

 but as soon as one revolution of the whorl is completed, the black 

 excretion of the hood replaces the outer layer to a considerable extent 

 on the dorsal or involved side. The outer layer is still present, but is 

 very thin. 



In Goniatites and Ammonites the same layers are present, but do 

 not extend around the dorsal side, except in the loosely coiled young 

 of certain species. 



In Nautilus, Goniatites, and Ammonites a thin layer lines the interior 

 of each chamber and coats the exterior of the siphon. This alone 

 appears upon the dorsal or involved portion of the whorls in the last 

 two groups. 



All the layers, with the exception of the black layer of the hood and 

 lining layer, betray everywhere an imbricated structure which shows 

 that they were deposited by the edge of the mantle from within. The 

 lining layer, and the black deposit of the hood are continuous through- 

 out wherever they occur. 



The siphonal coecum of Nautilus is formed by the funnel of the septum, 

 which projects posteriorly until it comes in contact with the shell of the 

 apex. The funnels of the younger septa are longer than those of the 

 older septa, and the sheath of the siphon which springs from the funnel 

 of the second septum extends posteriorly and lines the interior of the 

 siphonal coecum, forming a second siphonal coecum within the first. 



In Goniatites and Ammonites the coecum is formed in a similar 

 manner, but the latter is not lined, the siphonal funnel and sheath of 

 the second septum extending only far enough to close the opening in 

 the first septum. 



Subsequently the siphon is composed of the funnel and the sheath 

 which is continuous with it, but has a looser and more porous texture. 

 The sheath is discontinued at the opening of the funnel of the next 



