ECHINODERMA. CYSTIDEA 149 



come off from each side of the food-grooves. In this 

 genus there are only three rhombs, and they are of the 

 more highly-developed type called pectini-rhombs, which 

 differ from pore-rhombs in being surrounded by a raised 

 rim and in having the folds of the plate more pronounced. 

 In some other Cystideans of this group the brachioles are 

 found near the mouth only. 



Distribution of the Cystidea 



The Cystideans are comparatively rare fossils. They 

 range from the Middle Cambrian to the Carboniferous 

 Limestone, and attain their maximum development in the 

 Upper Ordovician. In the Menevian, Protocystis is found ; 

 this also occurs in the Tremadoc Beds, and with it Macro- 

 cy stella. In the Ordovician, Aristocystis, Ecliinosphaira, 

 Pleurocystis, Glyptosphcera and others are present ; in 

 the Silurian, Lepadocrinus, Pseudocrinus, and Placocystis, 

 In the Devonian there are fewer forms, and from the 

 Carboniferous only one or two genera have been recorded. 



CLASS III. BLASTOIDEA 



In the Blastoids (fig. 61) the body consists of a calyx, 

 usually with a stem ; but the latter is rarely found 

 attached to the calyx. The calyx may be spherical, oval, 

 pear-shaped, or bud-like ; in most cases it is formed almost 

 entirely of thirteen plates, arranged in a regular manner. 

 True arms are not present. 



Pentremites is the commonest Blastoid, and may there- 

 fore conveniently be taken as an example of the group. 

 Its calyx (fig. 62) has the following structure. The aboral 

 part is formed of a cycle of three plates — the basals (b), 

 two of which are alike, and the third smaller. Above 

 the basals is a cycle of five radial plates (r) ; these are 



