304 THE ECHINOIDEA 



There are more than two vertical series in each interambulacrum. There 

 is a masticatory apparatus, but no external gills. The order is therefore 

 endocyclic, gnathostomate, anectobranchiate, with simple ambulacral 

 plates, and having granulate interambulacrals. 



This order represents an offshoot from the main Cidarid stem. It 

 began in the Silurian, attained its maximum in the Carboniferous, and 

 became extinct in the Permian. There are three families in the order, 

 and these form an evolutionary series. All differ from the Cidaroida, 

 by having granular instead of unituberculate plates, which, by itself, 

 however, is not a character of ordinal importance. The main feature 

 is the great increase in the importance of the ambulacral areas, reminding 

 us of Bothriocidaris. This character is well developed in the two more 

 specialised families, but in the Palaeechinidae it is only just appearing. 

 Thus the family named is closely allied to the members of the Cidaroida, 

 and is separated from that order only as it marks the beginning of a 

 very remarkable type of Echinoid structure. 



FAMILY 1. PAL/EECHINIDAE. Melonitoida, in which the ambulacral 

 plates are essentially biserial (or in one case triserial). Most of the 

 plates are primaries, and the remainder long, narrow, demi-plates. The 

 plates of the test are rigidly attached. One row of interambulacral 

 plates passes on to the peristomial membrane. Genera Palceechinus, 

 M'Coy (pars), and Rhoechinus, Keeping; and perhaps also Perischocidaris, 

 Neumayr (syn. Homotoeclius, Sollas). The family is separated from the 

 Melonitidae, owing to the great difference in the characters of the 

 ambulacra; but it is regarded as the ancestral group from which that 

 family was derived. RJwechinus is the simplest genus, and includes those 

 with the pore-pairs in a single series. Palceechinus, which ranges from 

 the Silurian to the Carboniferous, includes those in which the pore-pairs 

 are biserial, and demi-plates occur (Fig. XVI. 1). 



FAMILY 2. MELONITIDAE. Melonitoida, in which the ambulacral 

 plates are all small, simple klasma-plates, which are multiserial in arrange- 

 ment. These form broad areas. The test is rigid. One row of inter- 

 ambulacral plates passes on to the pBiistomial membrane. This family 

 represents a marked advance on the previous one. The tendency 

 towards the crowding of the ambulacral plates and the reduction of many 

 of them into klasma-plates has made great progress. Genera Oligoporus, 

 Meek & Worthen, Carboniferous ; the plates agree in general character 

 with those of Palceechinus, but the ambulacral plates are quadriserial 

 instead of biserial. Melonites, Norwood & Owen, Carboniferous; the 

 process has gone further, and each ambulacrum consists of from six 

 to sixteen vertical series. 



FAMILY 3. LEPIDESTHIDAE. Melonitoida, in which the ambulacral 

 plates are small klasma-plates, multiserial in arrangement. The plates 

 of the test imbricate. None of the interambulacral plates pass on to the 

 peristomial membrane. This family is the extreme type of the Meloni- 

 toida, and represents a condition in which the plate arrangement becomes 

 most irregular. It includes the species with the greatest number of plates 

 in the ambulacra. The plates being thin and small, the test is necessarily 

 fragile, a danger to the animal obviated by the imbrication of the plates. 



