HYDROIDA. BALE. 



those at the ends of the pinnae generally much enlarged, 

 lateral and terminal apertures distinct. Cauline sarcothecae 

 larger than the laterals, two on the rachis at the base of each 

 hydrocladium. 



Goiiangial pinna replacing a hydrocladium, the first inter- 

 node bearing a hydrotheca. Corbula open, with about 15-20 

 strongly arched pinnules on each side, each springing from 

 a separate internode of the rachis and furnished with two 

 lateral series of long slightly curved opposite tubular sarco- 

 thecae ; the two proximal ones on the distal side of each 

 pinnule without corresponding ones on the other side, and 

 the first one often bifid ; nodes between the pairs of sarco- 

 thecge usually indistinct or wanting ; two sarcotheca> on the 

 rachis at the base of each pinnule. 



Colour. Dark brown to black. 



My suggestion of 1887 that A. divaricata and A. ramosa 

 would probably prove identical is confirmed by Billard's 

 examination of the type of A. ramosa, which he finds to agree 

 exactly with the description of A. divaricata. Busk in 

 separating them seems to have been mainly influenced by 

 differences in the habit of his specimens, but much variation 

 exists in this, as in other characters. Generally the ramifica- 

 tion is quite irregular, but specimens occur with all the 

 branches distinctly in one plane. 



Small specimens, up to about half an inch, may be mono- 

 siphonic ; the first supplementary tube is stouter than the 

 primary one, which it partially envelops, and from it spring 

 the branches, their first nine or ten internodes supporting no 

 hydrocladia, but each bearing in front a large sarcotheca. 

 The very dark colour which characterises the typical form 

 is due to the remains of the soft parts, and is readily removed 

 by liquor potassae. 



A very characteristic feature is the large size of the aperture 

 between the hydrotheca and the internode, which extends 

 from the base of the hydrotheca to the intrathecal ridge, and 

 from side to side of the hydrotheca. Owing to the size of this 

 aperture the intrathecal ridge, which borders it, is situated 

 further from the base than usual, and when, as is commonly 

 the case, the lateral sarcothecae are long, their bases become 

 coterminous with the ridge, hence, as Ritchie points out, the 

 septal ridge which subtends the bases of the lateral sarcothecae, 

 and that which is opposite to the intrathecal ridge, are one 

 and the same. The prolongation of the lateral ;-arcotheca> 

 back to the intrathecal ridge is, however, not invariable ; in 



