THE INTERAMBULACRUM. 65 



in this paper. Dr. Mortensen (1903) describe.s a very young Hypsiechinus (my Plate 3, fig. 6), 

 in which the interambulacrum consists of but a single plate in each of the five areas. In young 

 Echinus (Plate 3, fig. 5) Bury (1895) figures a similar single plate, and Loven (1874, Plate 17, 

 fig. 49) figures a young echinoid in which there is only a single interambulacral plate in each 

 area. This single plate, it seems, represents a single-column stage consisting of this one plate, 

 and may fairly be homologized with the plate at the ventral border of the interambulacrum of 

 Bothriocidaris. 



Loven (1892) showed that in young Goniocidaris (my Plate 2, fig. 1), also in Strongylo- 

 centrotus (Plate 3, fig. 11), and Echinus, at the ventral border of the interambulacrum there 

 is a single plate which lies dorsal to the continuous row of primordial ambulacral plates. This 

 plate is evidently the single first formed plate, and is homologized with the ventral plate of 

 the interambulacrum in Bothriocidaris. In the young of the Recent Echini mentioned, the 

 first plate is succeeded in the second row by two plates introducing the character of a second 

 column, which is the next stage in differential development of the interambulacrum. A similar 

 single first plate succeeded by two plates in the second row is shown by A. Agassiz (1904) in 

 young Salenia (his Plate 21, fig. 1), young Phormosoma (my Plate 3, fig. 10), and in young 

 Arbacia (his Plate 54, fig. 2). In later life, in regular Echini, as shown by Loven, this single 

 initial plate is usually resorbed by the encroachment of the actinostome, so that in the adult 

 there are two plates on the peristomal border, as in Cidaris and most regular Recent Echini. 

 Arbacia (text-fig. 227, p. 193) and Phormosoma (text-fig. 43, p. 80) are exceptions, for, as Mr. 

 Agassiz showed (1904, p. 54, Plate 41), the single plate is retained at the ventral border of the 

 adult in these genera. As Loven (1874) showed in the clypeastroids and spatangoids, the plate 

 is retained at the ventral border of the corona in most cases, as the corona is not resorbed by 

 the encroachment of the peristome. There are certain exceptions to this, as later discussed. 



Turning to Palaeozoic types, we find in Perischodomus (text-fig. 30, p. 70; Plate 64, 

 fig. 2) that there is a single plate ventrally succeeded by two plates in the second row, as in 

 young Cidaris, etc., but in the third row there are three plates, which introduces another stage 

 in differential development, structurally an advance on that seen in Cidaris and all modern 

 Echini. In the next row of Perischodomus there are four plates, and later a fifth column is 

 introduced. In this type we see that a form with five columns of plates in the adult is built 

 up by a series of stages starting with a single column represented by a single plate ventrally, 

 and the succeeding stages are represented by additional rows or zones of growth progressively 

 introducing new columns or new characters in differential development. 



In Hyattechinus beecheri (Plate 26) we find ventrally a single plate representing a single 



' In his text Dr. Mortensen (1903, p. 89) calls this plate a genital, but he wrote me that this was an error, and in the 

 second jjart of his Ingolf Eehinoida (1907, p. 172), corrects the error, saying that these plates are the first interambulaerals 

 and the genitals are discernible on the abactinal side. 



