1894.] Structure and Affinities of Heliopora cserulea, fyc. 299 



the same sklerotome and the basiventrals of the next previous sklero- 

 tome. Thus skleromere 50 is composed of a postcentrum = inter- 

 dorsal 50 + interventral 50, and of a precentrum = basidorsal 50 + 

 basiventral 49. The intermuscular septum runs obliquely across the 

 precentrum, or, in other words, the precentra are bi-protovertebral or 

 bi-myomeric, but not the postcentra. The precentra of the tail of 

 Amia are homologous with the " pleurocentra " in the tail of the 

 Jurassic Eurycormus, while Amia's postcentra are the same as the 

 " hypocentra " of Eurycormus. 



In Lepidosteus osseus, of which adult specimens and larvae of various 

 stages were examined, the combination of parts into one vertebral 

 complex is superior to that of Amia, because each vertebra belongs, 

 with its entire anterior half (interdorsal 50 + basiventral 50), to 

 myomere 50, and with its posterior half (basidorsal 51 + interventral 

 51), to myomere 51. In other words, the vertebral mass is equally 

 divided between two successive myomeres, or the myomeres have an 

 equal share of the skleromeres. The vertebrae are now truly bi- 

 protovertebral or bi-myomeric, each vertebra being composed of 

 a + b. 



XIX. " On the Structure and Affinities of Heliopora ccsrulea, 

 Pall., with some Observations on the Structure of Xenia 

 and Heteroxenia." By GILBERT C. BOURNE, M.A.. F.L.S., 

 Fellow of New College, Oxford. Communicated by- 

 Professor LANKESTER, F.R.S. Received May 30, 1894. 



(Abstract.) 



I have had the opportunity of making a renewed examination of 

 the structure of Heliopora, partly through the kindness of Professor 

 Ray Lankester, who gave me a very well preserved fragment of a 

 colony brought by Dr. S. J. Hickson from Talisse, Celebes. I have 

 also used some specimens which I collected and preserved in spirit in 

 Diego Garcia, and, in studying the dried corallum, I have had the 

 advantage of a large collection, originally the property of the late 

 Mr. G-eorge Brook, which Mrs. Brook has very kindly handed over 

 to me. 



All the specimens in my possession are referable to the only recent 

 species known, Heliopora ccerulea, but one of them belongs to the 

 variety tuberosa, Dana. 



The Soft Tissues. These form an even sheet, investing the surface 

 of the colony, interrupted here and there by the mouths of the polyps, 

 which are the only apertures opening on the surface. The structures 

 described below are entirely superficial, and there is no direct com- 



