DESCRIPTION OF PROTECHINUS STAGE. 235 



After the initial interambulacral plate 1' (figures 3 and 4), in the further 

 development of the interambulacrum we have more plates built, and in 

 all echinoids above Bothriocidaris more columns as well. The number 

 of columns acquired varies within wide limits, from 2 in Cidaris to 11 in 

 Lepidechinus rarisplnus, but the extent to which new columns are added 

 in the Palseechinoidea varies very much in different types, as shown in the 

 table, facing page 242. 



The early stage in which we find a single interambulacral plate, to- 

 gether with two ambulacral plates, in each area is so important that it is 

 desirable to give it a name, the protechinus * stage. The protechinus is 

 an early stage in developing Echini, belonging to the phylembryonic f 

 period, in which the essential features of the echinoid structure are first 

 evinced. It is a period in the developing young in which we find the 

 ambulacral areas consisting of 2 vertical columns of plates, the inter- 

 ambulacra consisting of a single plate, and in so far representing a single 

 column of plates. This stage is known in the young of Goniocidaris (figure 

 3, page 234), Strong ylocentrotus (plate 3, figure 8), Echinus, Spatangoids 

 and Clypeastroids, Oligoporits J (plate 6, figure 26), Melonites (plate 3, 

 figures 10, 11), Pholido cidaris (plate 9, figure 54) and Lepidechinus (plate 

 7, figure 42). This stage finds its adult representative in Bothriocidaris 

 (figure 4, page 234), which has 2 columns of ambulacral and 1 of inter- 

 ambulacral plates in each area throughout life. 



This protechinoid stage of echinoderms is comparable as a stage in 

 growth to a similar stage which is expressed in the protegulum of 

 brachiopods, the protoconch of cephalous mollusks, the prodissoconch 

 of pelecypods, and the protaspis of trilobites. 



The next stage in growth is marked by the introduction of the second 

 row of interambulacral plates. In all forms above Bothriocidaris this 

 stage has 2 plates in each interambulacrum succeeding the first initial 

 plate, excepting Tiarechinas, which has 3 plates. This period is still 

 generalized, and does not indicate to which great group of Echini the 

 developing young belongs, excepting in so far as it excludes Bothriocidaris 



* From TrpujTos, primitive, and exiVos, echinus. Tlais name comes rather close to Protoechinus, 

 which was given as a generic name by Austin. His genus is, however, included by Dr Duncan (9) 

 as a synonym o{ Palceechinus, so that it is not likely to be used in future ; also, the name of a stage 

 and a genus need not be confused, for from the context the identicy would be clear, and the stage 

 name would, moreover, be spelt with a small letter instead of a capital. 



t The phylembryo is a name given by Jackson to an early period in development, yet having 

 characters which render the embryo referable to the class or phylum to which it properly belongs. 

 Robert Tracy Jackson : Phylogeny of the Pelecypoda, Mem. Bost. Soc. Nat. Hist., vol. iv, no. viii, 

 1890. 



X In OUgoporus and Melonites the initial single plate is known only from angles for its i-eception 

 (see pp. 144 and 192); also in Melonites this early stage is complicated by the accelerated develop- 

 ment of the ambulacra, for instead of 2 columns, 4 columns exist at the protechinus stage 

 (plate 3, figure 11). 



XXVIII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



