68 EXPLANATION OF PLATE 44'. 



chambered alveolus in the Belemnite, (Fig. 7. b. b'.) 

 but has no Siphon. (Blainville.) 



Fig. 4'. Longitudinal section of the apex of the shell of 

 Sepia officinalis. This apex is composed of granular 

 calcareous matter (a.), alternating with conical horny 

 laminas, which expand laterally into the horny margin 

 (e.) (Original.) 



Fig. 5. Longitudinal view of Fig. 4. The apex (a.) re- 

 presents the apex of a Belemnite. The back of the 

 shell (e.) the dorsal part of a Belemnite; and the 

 alveolar portion (b. b'.) represents the internal cham- 

 bered shell of a Belemnite. (Blainville.) 



Fig. 6. Anterior extremity of the lamellas, or alveolar 

 plates, exposed by a longitudinal section in Fig. 5. 

 In the mature animal these lamellas are nearly 100 

 in number ; a few of them only are here represented. 

 These alveolar plates form the internal chambers 

 of the Sepiostaire, and represent the transverse 

 plates of the Alveolus in Belemnites, and other 

 chambered shells; but as the Sepiostaire has no 

 siphuncle, its chambers seem not subservient, like 

 those of the Belemnite, to the purpose of varying 

 the specific gravity of the animal ; the intervals be- 

 tween its plates are occupied by an infinite number 

 of thin winding partitions standing perpendicularly 

 between the lamellae. 



Figs. 6'. 6". Thin calcareous partitions winding between, 

 and supporting the alveolar plates of the Sepiostaire. 

 The sinuous disposition of these partitions increases 

 their efficacy in resisting pressure, on the same 

 principle, as in the foliated edges of the transverse 

 plates of Ammonites.* The sinuosity of the cal- 



* Dr. Fleming has accurately described the structure of these parti- 

 tions, as exhibiting' perpendicular laminae, waved and folded in brain- 

 like gyrations which occasionally anastomose. 



