J. E. Duerden — Jamaican Actiniaria : Part T. — Zoanthece. 349 



elongated nematocysts, showing a spiral thread, occur sparingly. The mesogloea 

 is thin, and small granular cell-enclosures are scattered throughout. 



The endoderm is a low band of cells resembling that of the mesenteries ; 

 zooxanthellse, small nematocysts, and a weak muscle are present. 



Mesenteries. — The mesenteries are brachycnemic in type, and the perfect ones 

 are arranged at about equal distances apart all round the oesophagus. Twenty-one 

 pairs are present in one specimen. The endoderm is thin and crowded with 

 zooxanthellse and small oval nematocysts. A parieto-basilar muscle and a verti- 

 cally arranged musculature occur on each side. The mesogloea is well developed 

 throughout. Towards the insertion of the mesentery into the body- wall it is 

 thrown into small irregular plaits or pennons ; still nearer it narrows a little. A 

 basal canal and numerous irregular vertical canals and cell-enclosures occur the 

 whole length of the mesenteries, continuous in places with those in the mesogloea of 

 the column-wall. The reflected ectoderm rarely occurs, but the mesenterial 

 filaments are met with as usual. Towards the base of the polyp the mesenteries 

 begin to unite with one another, and ultimately form a reticulum-like structure 

 filling the whole of the ccelenteron. 



Gonads. — No gonads were present in three examples sectionized. 



From the latest researches of Professor JVPMurrich, it appears that the West 

 Indies possess two species of Isaurus, one from Bermuda, identified by him as the 

 Isaurus tuberculatus, of Gray (1828), and another, the Zoanthus tuberculatus, of 

 Duchassaing (1850), obtained from the Bahamas in the Northrop Collection, and 

 previously collected from Guadaloupe and St. Thomas. In his Bermudan paper 

 (1889 a), M c Murrich, however, considered Gray's form as identical, not only with the 

 Bermudan examples, but also with the Z. tuberculatus. Owing to these later results, 

 and the specific name tuberculatus being occupied by both forms, he has followed 

 Andres and adopted the term Duchassaingi for the Bahaman examples and for those 

 known to Duchassaing and Michelotti. 



Professor Hacldon and Miss Shackleton (1891) have described as new, a form, 

 I. asymmmetricus, obtained by the senior author from Torres Straits. In doing 

 this they state (p. 68-4): — -"It is undoubtedly nearly allied to the Mammillifera 

 tuberculatus of M c Murrich. The specific differences are the lesser number and 

 greater size of the tubercles, though their diameter is about the same, and their 

 asymmetrical arrangement ; the height of our species is about double that of the 

 West Indian form." 



The specimens described above seem to me to unite in a very marked manner 

 the two West Indian and also the Torres Straits examples. I regard the differences 

 in the external appearance of the tubercles, transverse annulations, &c, as largely 

 dependent upon age and method of preservation. Even in the details of 



