222 ROBERT TRACY JACKSON ON ECHINI. 



columns of plates in an ambulacral area. These plates are wide and low with two pores each. 

 The pores are not wanting ventrally, as stated by Gregory (1897). There are apparently 

 eight or nine columns of irregular interambulacral plates in an area and the primordial inter- 

 ambulacral plates are in the basicoronal row. The peristome is covered with ambulacral 

 plates only, and the lantern, as shown by SoUas (1899; my Plate 18, fig. 5), has the usual features 

 of Echini. The anus is apparently eccentric in an interambulacrum, as shown by Spencer 

 (1904), but ocular and genital plates are unknown. An attempt has been made to correlate 

 the only species in this family with Asteroidea and to regard it as a primitive echinoid, which 

 I believe is unreasonable as considered under the description of the species. There is only 

 one genus and species {Palaeodiscus ferox) from the Lower Ludlow, Silurian of England (Plate 

 18, figs. 1-5), (p. 250). 



In the Echinocystidae the ambulacra consist of four columns of plates in each area, being 

 evidently an independent taking on of this character of many columns, which is a feature of 

 the Palaeozoic. There are eight columns of plates in an interambulacral area. The anus 

 apparently, as figured, is eccentric in an interambulacrum (Plate 20, fig. 1). There is only 

 one genus and species which is imperfectly known: Echinocystites pomum Wyville Thomson 

 from the Lower Ludlow, Silurian of England (Plate 16, fig. 4; Plate 18, figs. 6-8; Plate 20, 

 figs. 1-4), (p. 252). 



The order Perischoechinoida includes all the remaining and the best known Palaeozoic 

 genera. In this order the periproct is within the oculo-genital ring. While in many genera 

 there are two columns of plates in each ambulacral area, there is in other genera an excess of 

 this number up to twenty columns, and this is the one group of Echini characterized by extreme 

 development in this direction. The interambulacra rarely have as few as three columns of 

 interambulacral plates in an area, usually there are more, and as an extreme, as many as four- 

 teen occur in each area. This character of the ambulacra and interambulacra is the most 

 striking Palaeozoic feature. Coronal plates may be imbricate or not, varying with the families; 

 also the primordial interambulacral plates may be in the basicoronal row or resorbed, vary- 

 ing with families. Of course this is a rather nice point to determine as a specimen must be 

 perfect ventrally in order to ascertain it, so that the evidence may be taken from only a few 

 species, or even a single species in a family, and is open to correction with further knowledge. 

 The peristome may have many rows of ambulacral plates only, the more primitive character, 

 or may have the same with in addition some interradial non-ambulacral plates. Ocular plates 

 are typically all insert, but in a few cases one or more to all are exsert, foreshadowing the typical 

 Mesozoic and immature (also often adult) modern character. The genital plates are of moderate 

 size and usually have from two to many genital pores. The madreporic pores have been rarely 

 found, but usually are not distinguishable and perhaps were not existent in some species. 

 The Palaeozoic periproct is well known in this order, and is practically uniform throughout. 



