52 NIAGARA GROUP. 



reefs were formed, which may now be traced for many miles ; single masses were 

 several feet in diameter, and the beauty of their structure is not surpassed by any 

 of the corals which now abound in the ocean. Some of the species, too, were almost 

 world-wide in their distribution, as Halysites catenulatus, Heliolites pyrtformis, and 

 Favosites forbesi. It is famous, too, for its Echinoderms. The Cystideans commenced 

 their existence in Taconic times, as evidenced by the plate called Eocystites, and 

 reached the climax of their evolution aud development in this Group, and almost 

 suddenly disappeared from the face of the earth, a few small species only being 

 found in the Lower Helderberg and Lower Devonian, where the entire order be- 

 came extinct. Cystideans were marine animals, related to the Crinoidea. Some 

 were sessile ; others possessed a column and roots, by which they attached to other 

 objects; and others were free, and possessed a flexible column tapering to a point, 

 which could be used for attaching purposes. The head was globular, oval, pyri- 

 form, conical, cylindrical, or of any other shape, but always covered with an exter- 

 nal skeleton composed of polygonal calcareous plates, which are sometimes very 

 richly ornamented. The fracture of the plates presents the same crystalline struc- 

 ture as crinoidal plates do. In some species the number of plates and order of 

 arrangement remained constant throughout the life of the animal, the size of the 

 animal increasing by the growth of the original plates, which enlarged throughout, 

 instead of by addition to the edges. In other species the plates are not limited in 

 number, and have no order of arrangement; they increase in size, or new plates 

 are introduced, so as to destroy uniformity in different specimens in the same 

 species. In other species the dorsal side has a definite number of plates aud regu- 

 lar order of arrangement without any increase, while the ventral side has no order 

 of arrangement of the plates, and they increase in number to cover the increased 

 growth of the animal. There are usually two principal apertures, and often many 

 smaller ones through which the most important functions of the animal economy 

 were exercised. One of these is called the mouth, and is found on the side near 

 the base or near the apex. It is a curious fact that so important an organ as the 

 mouth occurs almost anywhere on the body of a Cystidean, but, of course, always 

 occupying the same position in each species. Another aperture, called the am- 

 bulacral orifice, occurs near the center of the upper part of the body, and between 

 the bases of the arms, when the species possessed such organs. The other apertures 

 are called calycine pores and pectinated rhombs. The calycine .pores served in 

 some manner to introduce water into the interior of the animal, but they bear little 

 resemblance to each other in different species, and one can form no adequate idea 

 of the system of circulation. Pectinated rhombs differ in number and position in 

 different species, and sometimes do not occur at all. Their function, too, is an abso- 

 lute mystery, except they furnished another medium of communication from the 

 exterior to the interior of the body. The Blastoidea commenced existence in this 

 Group by the appearance of Stephanocrinus, and became extinct in Carboniferous 

 times. The order Myelodactyloidea, another Echinoderm of very uncertain affinity, 

 seems to have been confined to this age. The development of the Crinoidea was 

 wonderful, no less than 15 genera making their first appearance, eight of which are 

 unknown in later rocks. The Graptolitidse here became extinct. 



§ 105. Holocystites occurs in Ohio, Indiana, Illinois, New York, and other 

 States. Twenty-five species have been defined, and none are known from higher 



