F. A. Bather — Growth of Cephalojjod Shells. 



447 



so rare and its shell so retrograde that no theor3'- has yet been built 

 on it. Examination of Nautilus gave rise to the Secretion-hypothesis, 

 shortly stated as follows : — As the animal grows, the adherent muscles 

 and the cincture gradually advance, the siphuncle lengthening in 

 proportion ; thus a cavity is formed between the last septum and the 

 surface of the visceral hump : this latter then deposits calcareous 

 matter, beginning at the sides of the shell and proceeding towards 

 the siphuncle round which it is continued backward. During the 

 advance of the animal the anterior portion of the mantle secretes 

 calcareous matter, which it deposits in successive layers on the margin, 

 of the aperture (Edwards and Wood). 



Microscopical examination of the fresh Sejoion has led Dr. E. 

 Eiefstahl to propose what may be called the Intussusception-hypo- 

 thesis : — That each new septum is absolutely developed from the 

 preceding, and is removed therefrom by growth of the intervening 

 zone of the outer shell wall ; which growth is effected, not by 

 apposition, but hj intussusception (The Sepion and its relations to 

 Belemnites, Pala^ontographioa, Bd. 32, 1886). Startling though this 

 be, the minuteness of the investigation demands attention. His paper 

 consists of (a) description of minute structure of the Sepion ; (/3) 

 conclusions as to its mode of growth ; (7) establishment of homo- 

 logies with the Belemnite, thus bringing those conclusions to bear 

 on external shells. 



Fig. .^ 



WftJLCM 





Fig, 1. Sagittal section, posterior end of young Sepia shell. Diagram after RiefstaU. 

 Fig. 2. Sagittal section, anterior margin of Sepia shell : the partitions not shoAvn. 

 Fig. 3. Development of sinuous partitions. Diagram after Hiefstahl. The thick- 

 ness of the free memhranes is exaggerated. 



(a) The Sepion is a complex of conchyolin membranes, impreg- 

 nated with lime in varying degrees : in accordance with the extent 

 and mode of calcification three parts are distinguished (Fig. 1.) — 

 i. Middle plate ; calcification slight and not extended to the margin ; 

 ii. Outer portion, dorsal; membranes parallel and overlapping, 



