2 9 6 



MOLLUSCA. 



b. Spirulirostra. The shell (fig. 265) in this singular genus 

 consists of a chambered portion or phragmacone coiled into a 

 spiral, the volutions of which are separated. This is lodged in 

 a pointed calcareous portion or " rostrum." The only known 

 species of this genus is found in the Miocene Tertiary. 



Fig. ?(>$. SJ>iriih 



Billardii. Miocene Tertiary. 



c. Beloptera. Shell consisting of a nearly straight chambered 

 portion or "phragmacone," perforated by a siphuncle, and 

 lodged in a pointed calcareous rostrum which is furnished with 

 lateral wings. Two species only of this genus are known, both 

 from the Eocene Tertiary. 



d. Belemnosis. This genus has been founded for the recep- 

 tion of an Eocene fossil closely resembling Beloptera, but differ- 

 ing in not possessing any lateral expansions. 



FAM. 5. SPIRULID^E. Shell nacreous, discoidal, composed 

 of volutions which are not in contact with one another. The 

 shell is divided into a series of air-chambers by curved shelly 

 partitions, pierced by a ventral tube or " siphuncle." The 

 entire shell corresponds with the " phragmacone " of the skele- 

 ton of the Belemnites. Spintlirostra and Beloptera are often 

 referred to this family ; but if these be placed in the Sepiadce, 

 the family of the Spirulidce is then without any known fossil 

 representative. 



FAM. 6. BELEMNITIDJE. Shell internal, composed of a 

 conical chambered portion (" phragmacone "), with a marginal 

 or ventral siphuncle, lodged in a cylindrical fibrous " guard," 

 and produced in front into a thin horny or shelly plate or 

 " pen " (the " pro-ostracum "). The Beleitmitida are exclusively 

 confined to the Secondary Rocks, ranging from the top of the 



