CLASS I 



ECHINOIDEA 



265 



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

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

 rows of ambulaci'al plates only (Bothrioci- 

 daris, Fig. 377, A ; Hyattechinus, Fig. i29, A ; 

 Fhormosoma, Fig. 371, A). These plates 

 after the first row have doubtless been de- 

 rived by migrating down from the coi'ona 

 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). Theu'e may be 

 scaly non -ambulacral plates only (Spatan- 

 goids), or the peristome may be quite naked 

 of plates (Clypeastroids). 



Aristotle's Lantern. — All Echini with the 

 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 canaliculata A. Agassiz. Re- 

 cent ; Falkland Islands. Young, 1.45 nim. in 

 diameter. Primordial ambulacral plates till 

 the peristome, primordial interambulaoral 

 plates in the basicoroual row succeeded by 

 two plates in the second row in each inter- 

 radial area (after Jackson). 



Fig. 371. 

 Characters of peristome and base of the corona in representatix'e Echini. A, Phurmosona pkwenta 

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

 B, EiicidarU tribuloides (Lamarck). Bahamas. On the peristome many rows of ambulacral and iu addition 

 interradial non-ambulacral plates. C, Archaeocidaris irortkeni Hall. Lower Carboniferous. Partially restoi'ed. 

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

 muUipnrus (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 

 many on the periphery of peristome in each area. E, Stro-ngylocentrotus drdhachiensis (O. P. MUUer). York 

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

 Echinocard i am flavescens {MiWler). Recent. On the peristome many non-ambulacral plates only. In figures 

 .1 and F the primordial interambulacral plates are in place in the basicoronal row ; in the other figures 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 comjDosed of two j^arts, 

 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 regular Echini ; or procumbent in 

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

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



