126 ACALEPHS IN GENERAL. Part I. 



concerned, the evidence of their acalephian nature is hardly less strong than that 

 adduced for the acalephian affinities of the fossil Tahnlata. The only difference 

 in the evidence is, that for the Tahnlata we have the confirmation of these affinities 

 in the structure of the animal of one of their living members, while that evidence 

 is wanting for the Rugosa. But, as I have already stated, the Coral stock of 

 the Rugosa coincides so flir Avith that of the Tahnlata, that it is built up with 

 successive floors, extending uninterruptedly across the bottom of the whole cavity, 

 which was evidently occupied by the animal; while the Coral stock of all genuine 

 Polyps presents radiating partitions extending uninterruptedly from top to bottom 

 of the cavity occupied by the animal, and the horizontal floors that may exist, 

 stretch only from one of these radiating partitions to the other. 



Now, if both Rugosa and Tahnlata are Acalephian Corals, it is very desirable 

 that correct views of their affinities with the other Acalephs should be obtained, 

 in order to arrive at correct conclusions respecting the order of succession of the 

 Acalephs in past geological times, and of their connection, through the only known 

 fossil Discojihorous Medusa, with the living representatives of the class. 



Scanty as is our information of the fossil Acalephs lu\yond the knowledge of 

 the order of the succession of the Rugosa and the Tabulata, it is already highly 

 intei-esting, even with these imperfect data, to institute comparisons between all 

 the memliers of the class respecting their order of succession. We have seen that 

 the Ctenophora3 are the highest order of Acalephs, and that the Discophorit proper 

 are next to these in standing; while the Hydroids, including the naked-eyed Me- 

 dusa and the Siphonophora?, constitute the lowest order of the class. We have 

 seen, further, that among the Hydroids themselves, those in Avhich the medusoid 

 elements prevail over the hydroid elements should be considered as the superior 

 ones. Taking, now, the only indicati(ni we have in Millepora as our guide to an 

 appreciation of the standing of the TaJjulata among the Hydroids, it is plain, from 

 the circumstance that these Hydra? conununities form large, permanent Coral stocks, 

 living proba1)ly for centuries, that they have a character of inferiority as con- 

 trasted with the short-lived Hydro-Medusaria, and especially with those which 

 produce free Medusa? in alternate generations. But if the Tabulata stand low in 

 the lowest order of Acalephs, we have in this fact a striking coincidence with 

 the character of the representatives of other classes in earlier periods. Since Crinoids 

 prevail in PaliBozoic times, while free Star-fishes and Echinoids make their appear- 

 ance later ; — since Bryozoa and Brachiopods prevail during the same old periods, 

 while Lamellibranchiates become prominent in later geological epochs ; — since 

 Trilobites are the earliest Crustaceans, followed by gigantic Entomostraca, and 

 higher Crustacea appear only in the middle geological ages, etc., etc., we should 

 expect that Acalephs also should make their appearance with the representatives 



