284 Rev. T. Hincks on new Hydroida and 



zooecia. Tlie exact position of Flustra solida is somewhat 

 difficult to determine. Smitt has placed it in his Escharella, 

 which is essentially equivalent to the genus Smittia, mihi ; 

 but from this group it differs in several important points. 

 For the present it must hold a provisional place. 



Suborder Ctenostomata. 

 Alcyonidium, Lamouroux. 



Alcyonidium excavatum, n. sp. (PI. XV. figs. 8, 9.) 



Zoarium minute (about | inch in height), erect, somewhat 

 clavate. Zooecia on one surface only, which is convex, the 

 other concave, hollowed out in the centre ; the cells irregular 

 in shape, the outlines indistinctly visible on the surface ; no 

 papillse. 



This interesting form is characterized by its minuteness, by 

 the disposition of the zooecia on one side only of the claviform 

 zoarium, and by the channelled condition of the opposite side. I 

 have only seen two specimens, which occur on the tube of an 

 Annelid : they are both of about the same size, and exhibit the 

 same characters ; and I see no reason to doubt that they are 

 adult organisms. The centre of the non-celliferous surface is 

 hollowed out from the top to within a short distance of the 

 base of the zoarium ; and the excavated portion is surrounded 

 by a narrow border. 



Arachnidium, Hincks. 



Arachnidium simplex , n. sp. (PI. XV. figs. 10, .11.) 



Zooecia disposed in linear series, elongate, expanded above, 

 and slightly attenuated downwards, prolonged below into a 

 delicate adherent fibre, by which they are linked together ; 

 the oral extremity turned obliquely upwards and free. 



Hab. On the stems and branches of Menipea. 



Though I have referred this form to Arachnidium, it differs 

 in one respect from the other known members of this genus ; 

 indeed the generic character must be modified to admit of its 

 reception. In the present species, so far as I have seen, the 

 zooecia are always arranged in simple unbranched linear series. 

 In the typical Arachnidia, on the contrary, branch lines are 

 given off from each side of the zooecia, and the zoarium is 

 more or less regularly reticulate. 



The present form has the closest affinity in general struc- 

 ture with Arachnidium ; and at present I do not see any suffi- 

 cient ground for detaching it from this group. 



