LECTURES ON MOLLUSCA. 163 



ing in a knob, and built up within with layer upon layer of very deli- 

 cate wafer-like shelly plates, supported by numerous vertical pillars.* 

 It is, therefore, very light and porous, at the same time that the shape 

 and texture of the back give it great power of support. The cuttles 

 are the least elegant of the tribe, having a large, flatish body, finned 

 along the whole of each side. The knob, doubtless, protects the 

 creature's tail from blows as it swims backward near the shore. The 

 Chinese cuttle bones are sometimes eighteen inches long. 



Most persons have seen the delicate Spirula, transparent and white, 

 shaped like a ram's horn divided across by pearly chambers. A mere 

 conchologist would never suspect any close resemblance between this 

 and the cuttle-bone. They are, however, so closely connected by in- 

 termediate fossil forms, that, without a knowledge of their animal, it 

 is difficult to say to which family these belong. No less different at 

 first sight are the "thunderbolt stones," so common in the Jurassic 

 and cretaceous rocks of Europe. In the world's history, they begin 

 and end with these rocks. They were suddenly poured, in incalcula- 

 ble abundance, on our planet ; and as suddenly they became entirely 

 extinct. The 



Family Belemnitid.e 



consisted of cuttles whose body was strengthened by a long pen, join- 

 ing on, at the tail end. to a conical chambered shell, the air-cells of 

 which were connected by a siphuncle at the side. This conical shell 

 (formerly called the alveolus of the belemnite, and now known as the 

 pkragmocone,) was invested, at the tail end, with a longer cone or 

 guard. This is fibrous, consisting of long prismatic cells, like the 

 shell of the recent pinnas or the great cretaceous Inocerami, with 

 which it entirely agrees in specific gravity. This guard is the "thun- 

 derbolt stone" of the common people, and is generally preserved 

 entire, while the chambers are often destroyed, and the pen has 

 almost always perished. The most perfect specimens were found in 

 the Oxford Clay, and are preserved in the British museum and in the 

 cabinet of Dr. Mantell. Fragments of the chambered part,- in the 

 Lias and Oolite, are very like the then-extinct orthoceratites, though 

 the animal is widely different. The last chamber alone sometimes 

 measures six inches by two and a half; so that its cuttle must have 

 been nearly three feet long. A fortunate breakage, in a specimen in 

 the British museum, displays an ink-bag near the siphuncle, at once 

 showing that it was an active swimmer, like the cuttles. The length 

 of the guard is very variable in the same species, sometimes attaining 

 to two feet. The septa frequently perish, leaving the chambers, 

 which have been filled with calcareous spar, lying loosely on each 

 other like a pile of watch glasses. 



The Belemnites were gregarious, and probably lived in a moderate 

 depth of water. The classical writers before Pliny gravely supposed 

 that they were the hardened contents of the bladder of the lynx; 



*This substance, when reduced to powder, is cniief. pounce. Among ether uses, when 

 rubbed on paper alter "scratching out," it prevents the ink from running. 



