CYSTOIDEA. 293 



here likewise a group of forms, in which no pores or fissures 

 have yet been detected in the test, though it is possible that 

 such really exist. Among these, two of the most remarkable 

 types are Holocystites and Gomphocystites, both from the 

 Upper Silurian (Niagara group) of North America, In 

 Holocystites (fig. 174, F) the calyx is long and sub-cylindrical, 

 and is composed of six or more ranges of hexagonal or poly- 

 gonal plates, which have a tuberculated surface, but are not 

 known to be perforated by any respiratory pores. A short 

 jointed stalk is present, but there is no evidence as to 

 the existence of arms or pinnulae. In Gomphocystites (fig. 

 175, F) the calyx has a very peculiar shape, being pyriform, 

 very narrow below, and inflated at its summit. The 

 integumentary covering consisted of numerous ranges of 

 plates, which were granulated superficially, but show no 

 signs of pores. There is, further, the character that the 

 summit is grooved by five sessile arms, which surround a 

 central opening, and have the same spiral arrangement as 

 in Agelacrinus. 



In another group of Cystideans the plates of the test are 

 indefinite in number, and are not arranged in a symmetri- 

 cally radial manner, but most or all of them are perforated 

 by respiratory pores. The type -genus of this group is 

 Echinosphcerites (fig. 174, A), in which the respiratory pores 

 are numerous, and are situated along the margins of the 

 plates, those of any one plate being united in pairs with the 

 pores of contiguous plates by canals which are visible exter- 

 nally. The genus has been quoted from the Devonian, but 

 appears to be truly confined to the Silurian period. Caryo- 

 cystites, of the Lower Silurian, is another close ally of the 

 preceding, but has an elongated and sub-cylindrical calyx. 

 In Sphmronites, on the other hand, though there are some 

 general resemblances to Echinosphcerites, there is the distinc- 

 tion that the pairs of connected pores are upon the same, 

 not upon different plates, the canals which unite them not 

 running across the sutures between contiguous plates. Both 

 Sphceronites and the allied Glyptosplicerites seem to be essen- 

 tially Silurian, and the Devonian fossils which have been 

 referred to the former will probably find a place elsewhere. 



