86 Revietvs — Nicholson and ZydckJcer's Palaeontology. 



relationship to the recent EeKopora. . It is, however, pointed out 

 that their minute structures are widely different, and that the 

 autopores or corallites generally possess a constant number of septa. 

 Other differences might also be mentioned, and there is room for 

 doubting whether the interstitial tubes in these old fossils were 

 tenanted by rudimentary polypes as is stated to be the case in 

 Heliopora ; for in the genus Plasmopora the intermediate tissue is 

 vesicular instead of tubular in character. In many respects the 

 Corals of these families show a relationship to Syringopora. 



Considerable advance in our knowledge of the microscopic struc- 

 ture of fossil Corals has been lately made, and it seems likely that 

 in any fresh classification the minute structure of the walls and 

 septa will have to be taken into account as an important feature in 

 the relationship of different forms. The significance of this is well 

 shown in the figures given on page 247 of transverse sections of 

 a recent CaryopJiyllia, and of two Palaeozoic Corals, Streptelasma and 

 Zaphrentis, in which the structure and arrangement of the septa are 

 essentially similar in all, though in the present accepted classification 

 the former genus is widely separated from the latter. 



Some important remarks are contributed on the nature of coral 

 reefs, and the difficulty is pointed out of determining whether the 

 outcrops of coral limestones in the older rocks really represent the 

 boundaries of coral reefs comparable to those now in process of 

 formation. 



A group of organisms, the Monticuliporoids, very numerously 

 represented in Palaeozoic rocks, and about whose affinities much 

 uncertainty rests, are treated in a separate chapter. These forms 

 have been more especially studied by the author, who regards them 

 as probably Corals, but other authorities consider them to be Polyzoa. 

 Waagen has lately stated that the mode of increase in some of these 

 forms is by ccenenchymal gemmation, like that of some undoubted 

 Corals, but there is reason to doubt the accuracy of this observation, 

 and it is likely that the mode of growth, in some of the Fistuli- 

 poridae at least, is similar to that in Polyzoa. 



The sub-kingdom of the Echinodermata is considered in the 

 Chapters xxii. to xxvi. Good figures are given of the minute 

 structure of the skeleton, so characteristic of the whole group, and 

 reference made to the distinctive cleavage which reveals at once the 

 presence of these organisms in the rocks. The group is ranged under 

 two divisions: the Echinozoa, which includes the Echinoids, Asteroids 

 Ophiuroids and Holothuroids ; and the Pelmatozoa embracing the 

 Crinoids, Cystideans and Blastoids. Important additions are made 

 in all these divisions in the present edition, more particularly in the 

 Crinoids and Blastoids. The former class is divided into Neocrinoids 

 and Palasocrinoids ; but owing to recent discoveries in the structure 

 of Encrinus and Taxocrinus, this arrangement will have to be 

 abandoned, and the classification of "Wachsmuth and Springer, 

 modified by Dr. P. H. Carpenter, will take its place. In this, the 

 Crinoids are divided into three orders : the Coadunata, Inadunata, 

 and the Articulata. In the Blastoids, the divisions into the two 



