RHIZOTROCHUS. 17 



Desmophyllum solidum Pourt. 

 Plate V., figs. 5 and 6. 



Corallum conical, pedunculate, heavy. Septa very thick, finely 

 granulated, in six complete systems and three cycles. The 

 primaries and secondaries are nearly equal, somewhat exsert, the 

 tertiaries are lower, but twice as thick ; all the septa are so 

 nearly in contact that the interseptal chambers are almost obliter- 

 ated, except near the middle of the calicle. Costee also very thick, 

 continuous with the septa, not continued far from the border of 

 the calicle. 



Height 11 millimeters, diameter eight. One specimen (in a semi- 

 fossil condition) found with the preceding species in 315 fathoms, off 

 Double-Headed Shot Key. There are several species of this type 

 with thick septa in the tertiary beds of Sicily, which have been 

 described by Seguenza ; none, however, appears to be identical with 

 this. 



RHIZOTROCHUS M.-Edw. & H. 



Rhizotrochus fragilis Poitrt. 



Rhizotrochus Jragilia Pourt. Bull. Mus. Comp. Zool., No. 7. 



Plate IV., figs. 1 to 4. 



Corallum simple, pedunculate, straight or slightly curved, regu- 

 larly conical. Calicle subelliptieal, deep. Six complete systems 

 of septa, four cycles. Septa very thin, not exsert, finely granu- 

 lated ; those of the first and second order meeting in the centre, 

 and united for about half their height. Costoa not prominent, no 

 epitheca. Hollow roots attached to the costoe of the second order 

 in pairs, at about one third or one half the height of the corallum, 

 and continued down to the peduncle, where they slightly expand; 

 they are never detached from the wall. The whole corallum very 

 thin and fragile. 



The young at first form a six-sided rounded cell, wider at the base 

 of attachment than at the top. The six primary septa reach only 

 half-way to the centre. At the next stage they unite in the 

 centre, frequently leaving a small open space in their middle. 

 Simultaneously or after the appearance of the secondary septa, a 



