STRUCTURE OF ECHINOIDS. 177 



pass across the boss of a tubercle without appreciable deflection. 

 Loven (Etudes) expressed a similar view to that of Duncan. 



A. Agassiz (Revision of the Echini) stated that compound 

 plates might be developed by the subdivision of original primaries, 

 but no other author seems to have accepted this view. 



Lambert, in 1892 (Bull. Soc. geoL France, vol. xx. pp. 38-100), 

 referred to plating-complexity, and ascribed it to the pressure of 

 new plates, not mentioning any influence of tvibercle-growth. 

 But in 1900, the same author (Bull. Soc. Sci. Yonne, vol. liii. 

 pt. 2, pp. 3-57) expressed the opinion that the chief cause of 

 " plate-crushing " was to be found in the expansion of the 

 tubercles, and that normal " gi'owth-pressure " exerted but a 

 secondary influence. 



Both Duncan and Lambert considered, then, that ambulacral 

 "plate-crushing" had a dual origin, the former holding that the 

 pressure of new plates was the main cause, the latter regarding 

 tubercle-growth as the chief agent. 



More recently, Bather (1909, ' Triassic Echinoderms of Bakony ') 

 has suggested that the oncoming of " plate-crushing " in. the 

 Diademoid (Centrechinoid) ambulacrum is to be correlated with 

 the growth of the perignathic ambulacral processes, which restrain 

 the plates froixi passing freely to the peristome margin. This 

 view is, in eflfect, a return to that of Duncan, that growth-pressure 

 of new plates is the main cause of the development of com- 

 pound ambulacrals. Jackson (1912, ' Phylogeny of the Echini') 

 considers " growth-pressure " alone in his discussion of ambulacral 

 structures. 



In considering the corresponding features among the Holecty- 

 poida, I have lately taken the same view (Geol. Mag. 1910 & 

 1911), and, as the structures above described in Echinocardiutn 

 cordatum seem to have a bearing on the problem, I propose to 

 give a discussion of the subject. 



VL The Origin of ^'- Plcde-crushing." 



The ambulacra contain compovmd plates in all groups of Regular 

 Echinoids except the Cidaroida. Excluding the highl}^ specialised 

 and aberrant Perischoechinoida, the Regular Echinoids may be 

 separated into two sections, in one of which (the Cidaroida) 

 resorption of old plates at the peristome takes place freely and 

 continuously, so that the proportionate diameter of the peristome 

 remains constant throughout growth ; and in the other (the 

 Centrechinoida) the resorption does not keep pace with the 

 growth of the corona, so that the peristome of an adult is pro- 

 portionately smaller than that of a young foi'm. In all the 

 Cidaroida, and almost all the Centrechinoida, the growth of new 

 plates at the edge of the oculars is continuous and regular. Thus 

 while the Cidaroid ambulacrals can pass freely down the full 

 length of their area, those of the Centrechinoid are to some degree 

 hindered. The result must obviously be a compression of the 

 earlier ambulacrals against the obstacle (e.g., Jlemipedina, iind, 



