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RADIATA. 



CLASS II.— ACALEPHJE. 



These soft, gelatinous Kadiates, known as " Jelly-fishes," " Sea- 

 nettles," " Sea-blubbers," " Medusae," etc., vary in size from an al- 

 most invisible dot to a yard in diameter. Large ones often weigh 50 

 lbs. ; yet they are little more, as it were, than " coagulated water," for 

 when dried, nothing is left but a film of membrane, thin as gossamer? 

 and weighing but a few grains. The animal is covered by a very delicate 

 epidermis, under which are situated the nettling capsules. There is a 

 distinct muscular system ; but the only motion observed consists of an 

 alternate contraction and dilatation of the disc, performed with great 

 regularity about fifteen times a minute. 



Acalephs have left few traces in stratified deposits, owing to the ex- 

 traordinary softness of their bodies. According to Agassiz, Favosites 

 and Ohsetetes belong to this class ; and it is quite possible, says Dana, 

 that most of the so-called Graptolites are Acalephs. 



No. 962. Acalepha deperdita, Bey. 



Two specimens. The remains of this frail creature are circular imprints, 

 composed of very regular concentric circles, on the borders of which are eight 

 rays. These are the only sure remains of a true Acaleph which have been found 

 fossil. From the Lithographic limestone (Middle Oolite\ Eichstadt, Bavaria. 



Price, $0.75- 



No. 963. G-raptolithus octobrachiatus, Hall. 



The Graptolites were first considered of 

 vegetable origin ; subsequently they were re- 

 garded as extremely slender Orthoceratites ; 

 but Portlockhas pointed out their analogy with 

 Sertularia or the lowest forms of Acalephs. 

 They consist of sessile polype cells arranged 

 in one or two rows on flexible, tubular stems 

 which radiate from a central disc. The speci- 

 mens usually observed are fragments of stems 

 in a flattened condition, presenting only a 

 serrated edge ; they seldom preserve more of 

 their substance than a carbonaceous or cor- 

 neous film of extreme tenuity. Graptolites 

 are exclusively and characteristically Silurian 

 fossils, and prevail in argillaceous deposits. 

 This specimen is an artificial representation 

 (it being quite impossible to take a cast of the 

 delicate original) of an eight-armed Grapto- 

 lite from the Quebec group as figured by 



