IMBRICATION OF CORONAL PLATES. 73 



plates at the ventral border of the interambulacrum and two in the ambulacrum (Maccoya, 

 Plate 33, fig. 1; Lovenechinus, Plate 43, fig. 1; Oligoporus, Plate 50, fig. 8), or two in the 

 interambulacrum and four in the ambulacrum (Melonechinus, Plate 56, fig. 2). Resorption 

 may have cut away more than one row, probably three or more rows, when we have the condi- 

 tion in Archaeocidaris (Plate 9, figs. 7, 8; Plate 10, fig. 10), where there are four plates in 

 the interambulacrum and two in the ambulacrum in the basicoronal row. 



Resorption may cut away an indeterminate number of plates, when we find the condition 

 seen in most modern regular Echini, as Cidaris and Strongylocentrotus, with two interambula- 

 cral and two ambulacral plates in each area in the basicoronal row. This includes, I believe, 

 the essential structure of the basicoronal row of all Echini. The proximal modifications of 

 these plates by the development of the perignathic girdle are considered under that head (p. 190). 



The enlargement of the peristomal aperture in Echini is attained l^y two methods: by 

 the growth of the individual plates, which will enlarge the opening to any extent ; or by cutting 

 away the plates by resorption, which is a more obvious method of enlargement. The growth 

 of the plates is, however, the more important factor. Even when resorption has not cut away 

 any interambulacral plates, as in Echinarachnius, Spatangus, Arbacia (text-fig. 227, p. 193), 

 and especially Phormosoma (text-fig. 43, p. 80), the size of the peristomal opening is many 

 times larger than it was in the young individual. 



Imbrication of Coronal Plates. 



In Echini, in many types, there is no imbrication in the coronal plates, the suture lines of 

 the plates being so nearly perpendicular to the surface that the sides of a plate are as nearly 

 parallel as possible in a curved test. The sides of a plate may be far from -parallel where the 

 plates are very thick, as in Lovenechinus nobilis (Plate 38, fig. 9), but still they are perpendicular 

 to the surface and are therefore not imbricate. 



Certain plates in Echini may present a strong angle on the suture line in a given direction, 

 which Bather suggestively calls beveling. Beveling is applied as a term where the inclination 

 is in a limited number of plates only. An excellent example is in the adradial suture of the 

 Palaeechinidae. Here in all species of the family, as shown in Lovenechinus septies (Plate 45, 

 fig. 5), the ambulacral plates are inclined over the adjacent interambulacral plates, so that a 

 strong bevel results in the plates concerned. A similar bevel is seen in the petaloid area in 

 some clypeastroids (Clypeaster), where the ambulacra are strongly inclined over the interam- 

 bulacra on the adradial suture. 



In Clypeaster rosaceiis a curious combination of direction of beveling occurs. As stated, 

 in the petaloid areas, the ambulacra bevel over the interambulacra as they do in the Palae- 



