CLASS I 



ECHINOIDEA 



265 



which are found in place and fill the area (Fig. 370). From this primitive 



condition various departures exist. The area may be filled with two or many 



rows of ambulacral plates only {Bothrioci- 



daris, Fig. 377, ^ ; Hyattechinus, Fig. 429, ^ ; 



Phonnosoma, Fig. 371, A). These plates 



after the first row have doubtless been de- 



rived by migrating down from the Corona 



as shown by Loven. There may be many 



rows of ambulacral with interradial non- 



ambulacral plates (Cidaroida, Fig. 371, B; 



Archaeocidaris, Fig. 371, C ; Melonechinus, 



Fig. 371, D). There may be one row of 



ten primordial ambulacral plates with more 



or less solid, scaly, or isolated non-ambu- 



lacral plates, or rarely no non- ambulacral 



plates (most Centrechinoida). There may be 



scaly non -ambulacral plates only (Spatan- 



goids), or the peristome may be quite naked 



of plates (Clypeastroids) 



AristoÜe's Lantern 

 exception of Spatangoids (and possibly some 

 Holectypoids) are provided with a powerful 

 masticatory apparatus known as the Aristotle's lantern, which, with the muscles 

 and their points of insertion in the test are of high systematic value. The 

 lantern is composed of forty pieces in all Echini except Clypeastroids. * There 



Fig. 370. 



Goniocidaris caTUÜiculata A. Agassiz. Re- 

 cent ; Falkland Islands. Young, 1.45 mm. in 

 diameter. Primordial ambulacral plates tili 



All -n T_' • 'J.I, j.1^ •tli'^ peristome, primordial interambulacral 

 -AU lliCnini Wltn ttie plates in the basicoronal row succeeded by 



two plates in the second row in each inter- 

 radial area (after Jackson). 



Fk!. 371. 

 Characters of peristome and base of the corona in representative Echini. J, Phormosoirm plucenta 

 Wyville Thomson. Recent ; otf Cape May to Cape Sable. On the peristome many rows of ambulacral plates. 

 />', Kucidaris tnbuloides (Lamarck). Bahamas. On the peristome many rows of ambulacral and in addition 

 interradial non-ambulacral plates. C, Archaeocidaris tnortheni Hall. Lower Carboniferous. Partially restored. 

 On the peristome many rows of ambulacral and in addition interradial non-ambulacral plates. /), Melonechinus 

 multiporus (Norwood and Owen). Lower Carboniferous. Restored. On the peristome many rows of ambulacral 

 and in addition two rows of interradial non-ambulacral plates ; ambulacrals pass from two plates orally to 

 manv on the periphery of peristome in each area. E, Strongylocentrotus dröbachiensis (O. F. Miiller). York 

 Harbor, Maine. On the peristome one row of ambulacral and scattered, small, non-ambulacral plates. ±, 

 Krhinocardium flavescens (Müller). Recent. On the peristome many non-ambulacral plates only. In hgures 

 A and F the primordial interambulacral plates are in place in the basicoronal row ; in the other hgures they have 

 been resorbed, with or without additional plates (after Jackson). 



are five teeth ; five pyramids, each composed of two halves, joined by suture ; 

 ten epiphyses ; five braces ; and five compasses, each composed of two parts, 

 joined by suture. The lantern is inclined, subtending an angle of about 

 90", in the young of modern and adult of Paleozoic Echini ; erect with sides 

 approaching the vertical in most Recent regulär Echini ; or procumbent in 

 most Clypeastroids. Teeth are grooved (Paleozoic genera, Cidaroida and 

 Aulodonta); or have a keel on the inner face (Stirodonta, Camarodonta, 



