454 



PELMATOZOA. 



aperture which has been usually regarded as a genital pore. The 

 calyx was furnished with a stem composed of thin annular joints. 

 The genus Sphczronites, also Ordovician in its range, is allied to 

 the preceding in several points, but there 

 was no stem, and the calyx was adherent 

 by its base to foreign bodies. In the 

 Silurian genus Gomphocystites (fig. 324) the 

 calyx has a very peculiar shape, being 

 pyriform, very narrow below, and inflated 

 at its summit. The calyx-plates are in 

 numerous cycles, and have superficial 

 granules, along with yoked pores. The 

 mouth is central, and is surrounded by 

 five spirally wound ambulacral grooves. 

 In Holocystites, again, the calyx (fig. 325, 

 f) is long and subcylindrical, and is com- 

 posed of six or more ranges of hexagonal 

 or polygonal plates. The mouth is cen- 

 tral, and the calyx was furnished with 

 a short stem, or was sessile. The genus is 

 found in the Silurian rocks of North 

 America and Europe. 



Order 3. Rhombiferi. — The Cysti- 

 deans included in this order are charac- 

 Fig. 324.— side-view of the cast terised by the possession of pores ar- 



01 the calyx of Lromfihocystites . * x 



gians, of the natural size, from ranged in rows, corresponding rows in 

 SiSSSSSTaSg3S adjoining plates being connected, exter- 

 nally or internally, by grooves or slits 

 which cross the lines of suture between the plates. The pores are 

 thus arranged in groups, which are usually more or less lozenge- 

 shaped, and which are known as "pore-rhombs." The pore- 

 rhombs were probably connected internally with organs similar to 

 those which will be described in dealing with the Blastoids as 

 " hydrospires," and their function seems to have been certainly 

 respiratory. 



In one section of this order the plates are indefinite in number, 

 and the pore-rhombs are distributed indifferently over the whole 

 calyx, and are exceedingly numerous. A good example of this 

 section is the well-known Ordovician genus Echinosphcerites (fig. 325, 

 a), in which the calyx is of large size, globular, and stemless, its 

 base being attached to foreign bodies. The calyx-plates are very 

 numerous, polygonal, with pore-rhombs on every side (fig. 321, c). 

 At the apex is the mouth, and the anus is lateral and is protected 

 by a large " valvular pyramid," while a small ovarian (?) opening is 

 placed between the two. Caryocystites, also Ordovician, is nearly 



