PRESIDENT'S ADDRESS. Ixvii. 



a spine or joint, the growth being from first to last in perfect 

 crystalline continuity." (Sorby.) It is divided into seven 

 primary groups, of which Cystoidea and Blastoidea are extinct, 

 Crinoidea nearly so. All have jointed calcareous stalks, by which 

 they can attach themselves to any support. The Echinoids, 

 Asteroids, Ophiurids, and Holothuroids are not stalked during any 

 part of their lives. The Cystoidea are intermediate between the 

 Echinoids and Asteroids on the one hand, and between the 

 Crinoidea and Ophiuroidea on the other ; they appear to have 

 become extinct before the appearance of its sub-family 

 Pentacrinus, and combine some of the distinctive characters of 

 each of the groups. 



Pentacrinus appeared for the first time in the Trias beds. 

 P. Caput-Medusce lives now in the West Indian Seas, and is the 

 only survival of this multitudinous family. It passes its whole 

 life attached to some object ; probably it had the power of 

 detaching itself, and moving from place to place in search of a 

 safer and more appropriate support. It is abundant in the Lias, 

 and now reduced. The Encrinites have no living representatives ; 

 they appear in many respects to have been of a lower organisa- 

 tion, and connected with the true Zoophytes, through the 

 Echinodermata. The Asteroidea or Star-Fish, represented by 

 Pakmster in the early Cambrians passes successively through 

 all the geological beds to the present day. The Holothuroidea 

 or Sea-Cucumbers which have no exoskeleton are furnished with 

 isolated plates, distributed throughout the epidermis, which is 

 soft, and capable of extension and contraction. The disconnec- 

 tion of these plates allows the utmost freedom of motion. 



The Molluscan Order is divided into two branches, the 

 Molluscoidea and Mollusca ; the former includes the Tunicata, 

 the Bryozoa and the Brachiopoda. These three are sufficiently 

 distinct to separate them from the true Mollusca. They are all 

 aquatic, and to a great extent marine. The Bryozoa, the lowest 

 of the group, strongly resemble the Zoophytes, both in general 

 structure and habits of life, so much so, that until lately they 

 were considered to belong to that Order. The propagation of 



