MOXOST^CHAS DICHOTOMA. 37 



with entire margin, associated with a pair of supracalycine nematophores^ 

 and with free mesial nernatophores. 



Gonobome. — Gonangia not contained in corbulae, or connected with special 

 branches. 



The unilateral disposition of the hydrothecal or ultimate ramuli on the 

 main branches is very remarkable, and, being absolutely constant, becomes 

 a character of generic value. 



Monostsechas dichotoma. 

 PI XXII. Figs. 1-5. 



Trophosome. — Hydrocaulus attaining a height of about an inch and a half, 

 pelhicid, dichotomously branched ; every alternate internode of the hydro- 

 thecal ramuli carrying a hydrotheca and separated from the others by a 

 transverse distal joint, and a very oblique and more strongly marked 

 proximal one. Hydrothecae rather large cup-shaped, flanked on each 

 side by a prominent tooth-like process of the internode. Supracalycine 

 nematophores borne by the tooth-like processes, one mesial nematophore 

 borne by the hydrotheca-bearing internode at the proximal side of the 

 hydrotheca, and two by the intervening internode ; numerous nemato- 

 phores borne in a single series along the opposed sides of the bifurcat- 

 ing branches. 



Gonosome. — Gonangia pyriform, contracted below into a short stalk, which 

 springs from a slightly prominent process of the internode, just below the 

 base of the hydrotheca, and carries a nematophore on each side of it. 



Off Pacific Reef, from a depth of 283 fathoms. 



The present species closely resembles, in several important characters, 

 the Plmmilaria caiharina of Johnston. In the form and position of the 

 hydrothecae, in the internodes and nematophores of the ultimate ramuli, 

 and in the form and position of the gonangia, the resemblance is so close, 

 even in minute details, that it is impossible to find in these parts any 

 characters by which the one hydroid can be distinguished from the other. 



It is entirely different, however, with the ramification of the species for 

 which I have here founded the genus Monost^chas. This ramificati'^^n is 

 of a very remarkable kind, so much so, indeed, that I regard it as affording 

 a character of generic value. It has not only no resemblance to that of 

 Plumularia caiharina., but belongs to a type which has no representative 

 in any other known Plumularidan. 



