CYSTIPHYLLOIDEA. 



30: 



an operculum of four calcareous plates, of which the dorsal and ventral 

 ones are trapezoidal and the lateral ones are triangular. 



In Rhizophyllum the conical or pyramidal corallum is attached to 

 foreign objects by root-like exothecal processes developed from its flat- 

 tened under surface, the upper surface being rounded. The calice has 

 septal striae and a distinct fossula, the internal structure is wholly of 



Fig. 182. — A, Goniophyllum pyj-amidale, from the Silurian of Gotland, showing the operculum 

 in place, twice the natural size ; b, Transverse section of the same species, slightly enlarged, (a is 

 after Lindstrom ; b is original.) 



oblique vesicles, and the operculum is in the form of a single semicircular 

 plate. The species of the genus are mostly Silurian, but one form is 

 found in the Lower Devonian. 



Still more closely allied to Cystiphyllum is the genus Arceopoma, com- 

 prising only the A. {Cystiphyllum) prismaticum of the Silurian rocks. 

 The corallum in this form is subangular, more or less clearly four-sided, 

 the calice having radial rows of septal tubercles, and the surface exhibit- 

 ing distinct rugae. Internally the structure is wholly vesicular, and the 

 calice is closed by an operculum of subtriangular valves. 



Lastly, in the genus Calceola (fig. 183), the slipper-shaped corallum is 

 free, and has the form of a slightly curved four-sided pyramid, of which 

 one side is broad and flat, the opposite side being 

 equally broad but convex, while the lateral faces 

 are much reduced, and are only represented by 

 the rounded junctions of the two broad faces. 

 The calice is deep, corresponding in form to the 

 exterior, and marked internally with distinct 

 septal striae. The " cardinal septum " is clearly 

 marked, and is placed in the centre of the vaulted 

 side of the corallum, sometimes in a distinct 

 groove or fossula, while the septa on both sides of 

 this have a pinnate arrangement. The " counter 

 septum" is in the middle of the flat side of the 

 corallum, and is also often in a distinct fossula. 

 The calice is closed by a half-moon-shaped oper- 

 culum, which resembles the valve of a Brachi- 

 opod in form and sculpturing, its under sur- 

 face, however, showing a prominent median and fainter lateral septal 

 ridges. The internal structure is dense, the thick wall apparently solid, 

 but showing, according to the observations of Kunth, a finely vesicular 

 structure. The only known species of this genus is the Calceola sa?idalina 

 of the Middle Devonian rocks, an abundant and characteristic fossil in 



Fig. 183.— The corallum of 

 Calceola sandalina (a small 

 example), from the Middle 

 Devonian of Gerolstein, view- 

 ed from its rounded side, of 

 the natural size. 



