1730 THE VOYAGE OF H.M.S. CHALLENGER. 



family, the Coelograpliida, and differ from the preceding family, the Concharida, in 

 which the siliceous wall of the two valves is much thicker, and perforated by regular 

 circular or roundish pores. 



The galea or conical cupola in the a^^ex of the two valves (" der kegelformige 

 Aufsatz " of the German authors) has in all Coelodendrida a triangular base and an 

 irregularly conical or nearly three-sided pyramidal form. Its cavity is about one-third 

 or one-fourth as large in diameter as the cavity of the hemispherical valve upon which 

 it rests. The galea is relatively smaller and more irregularly formed than in the 

 Ccelographida, and differs essentially from that of the latter in the constant absence of 

 a rhinocanna ; there are also wanting, therefore, the characteristic frenula, which 

 connect the nasal tube with the apex of the galea. The cavity of the galea probably 

 always communicates with that of the valves by pores in the separating siliceous plate, 

 and is besides pierced by irregular pores in its outer wall, very variable in form, size, 

 and number, but it does not communicate with the cavity of the hollow radial tubes, 

 from which it is separated by a thin, solid, siliceous plate. 



The hollow radial tubes which arise from the galea in the Coelodendrida do not 

 seem to possess that constant regularity in number, origin, and disposition, which 

 is found in the following family, and there serves for distinction of genera. In my 

 first description of the Coelodendrida (1862, loc. cit., p. 362), I pointed out this 

 irregularity, and mentioned that the number of radial tubes arising from each galea 

 varies from three to eight ; the total number therefore amounts to from six to 

 sixteen, the same minimum and maximum numbers which we shall encounter also in the 

 radial styles of the following family. But whilst it is easy to determine the position 

 and relation of these hollow tubes in the Ccelographida, owing to the constant sagittal 

 position of their rhinocanna, this task is very difficult in the Coelodendrida, where the 

 rhinocanna is wanting. In the most frequent cases there arise from each galea three or 

 four tubes, more rarely five or six, and very rarely seven or eight. The simplest and 

 probably the original case is the development of three tubes, two of which are paired 

 (divergent on the right and left), while the third is odd, lying in the sagittal plane. 

 Perhaps these three primary tubes may be compared to the three cortinar feet of the 

 Nassellaria, so that we may regard the two paired anterior as pectoral, and the odd 

 posterior as a caudal tube. Usually the two paired or pectoral tubes arise from two 

 corners of the triangular base of the galea, whilst the third odd or caudal tube does not 

 arise from the third corner of the base, but more or less above it, and often even from 

 the highest point or the apex of the galea. In the majority of species observed, this 

 odd sagittal tube is forked even at its origin, so that two divergent tubes (an anterior 

 and a posterior) arise from the apex of the galea (PL 121, figs. 3, 8). More rarely 

 the two paired or pectoral tubes are also forked at the base, so that three pairs of tubes 

 arise from each galea, and the total number of tubes amounts to twelve. Very rarely 



