THE AMBULACRUM OF THE CORONA. 53 



plates of the corona and in cases by resorption of the base of the corona. The plates of the 

 peristome are various in origin. The primordial ambulacral plates of regular Echini are appar- 

 ently in this area from their inception (Plate 3, fig. 7). Additional ambulacral plates are 

 derived by flowing down from the corona (text-figs. 41-48, p. 80). All other peristomal plates, 

 it is believed, are formed on that area and are not derived from the corona (text-fig. 57, p. 84). 

 These matters will be considered in detail in their appropriate places. 



The Ambulacrum of the Corona. 



The ambulacrum is the most essential feature of a sea-urchin, and has a first importance 

 in classification and morphology, on account of the varied structure that it presents. We can 

 conceive of a sea-urchin without an interambulacnini, although such is not known, or wanting 

 in almost any other skeletal parts, but it seems that an ambulacrum is an essential character 

 of the class. The detailed structure of the ambulacrum in post-Palaeozoic types is for the most 

 part fairly well known, but the Palaeozoic types have not been closely studied, and throw a 

 great deal of light on the structure of this area in the group as a whole. 



The ambulacrum is essentially an area for bearing tube-feet, which are important as a 

 means of locomotion in most Echini, or are modified more or less fully as organs of respiration 

 in other Echini (clypeastroids and others). The ambulacrum is never composed of less than 

 two vertical columns of plates, and when more than two, it always has an equal number on 

 each side; therefore there is a distinct median suture with an equal area on each side of it. 

 From this it follows that half an ambulacrum, barring exceptional variation, is as good as a 

 whole one for studj^, which is important in fossils where one half is often all that is available. 

 A partial exception to the rule of never less than two columns of ambulacral plates occurs in 

 Palaeolropus josephinae Loven, as figured by Loven (1874, Plates 13, 32). In this exception, 

 dorsally in all five areas the two columns drop out to a single column for a distance of three to 

 seven plates from the oculars. This indicates that a sea-urchin might have a single column of 

 ambulacral plates throughout at least the greater part of an area, but such is unknown. The 

 width of the ambulacrum varies greatly. It is narrow usually, in types with low imiserial 

 plates, Eucidaris (text-fig. 4), Palaeechinus (text-fig. 9) ; but it may be wide, as in the petaloid 

 area of Clypeaster. The ambulacnmi is wider usually when the plates are compound, Strongy- 

 locentrotus (text-fig. 5si), or when four colunms, Lovenechinus (Plate 45), or more columns, 

 Melonechinus (Plate 57), are developed. It may be much wider than the interambulacrum, 

 Lepidesthes colletti (Plate 70, fig. 1), Meekechinus (Plate 76, fig. 1), and about twice the width 

 of the interambulacrum in Bothriocidaris (Plate 1, fig. 1). 



In the primitive type, Bothriocidaris archaica (Plate 1, fig. 1), the ambulacral plates are 

 high, hexagonal, and there are therefore few tube-feet, about sixteen, to each area excluding 



