Morphological Results. 
253 
paths of development. From their peristomial skeletal structures, those of the primi¬ 
tive Triassic Diademoids still present no great difference. The differences that so 
soon afterwards appeared may have been a consequence of these different responses 
to pressure. Since in the Cidaridae the relief of pressure took place internally, the 
coronal plates continued as heretofore to pass on to the peristomial membrane, and 
if the ambulacrals, as Loven held, migrated downwards more rapidly, they would 
have been the less fitted to Support internal processes. In the Ectobranchiate line 
of descent, on the other hand, the evagination of the peristomial membrane at the 
edges of the interambulacra checked the passage of interambulacrals entirely, and 
also affected the passage of the ambulacrals, which became in Loven’s phrase the 
«dissolved wrecks» of their former selves. 
If this account be accepted as approximately correct, we may consider vvhat 
effect such changes would have on the perignathic girdle. In the Cidaridae the 
large internal projections (auricles) for the attachment of the pyramid or maxillar 
muscles are confined to the interambulacrum ; to each auricle is fixed a protractor 
muscle on its interradiad margin, and a retractor on its upper and radiad margins. 
In consequence of this proximity of their respective external attachments, there is a 
torsion of the muscle-flbres as the muscles bend round each other to reach their 
attachments on the maxillae; this is well shown in Loven’s figures (1892, «Echi- 
nologica», pl. vi, figs. 39—42). There is therefore a tendency for the auricles to 
diverge from the interradius over the ambulacra, and this is more marked in modern 
Cidarids than in the Triassic specimens here figured (Pl. VI, figs. 130, 142, 143, 
147, 148). When the passage of the coronal plates on to the peristomial membrane 
became checked by the extrusion of branchial sacs, it became possible for the 
perignathic attachments of the retractors to assume a more mechanically advanta- 
geous position nearer the radii, since projections for their fixation could now more 
readily be formed upon the ambulacra (Loven, 1892, figs. 47, 48). Loven considers 
that the adradially placed auricles of the Ectobranchiata are in fact derived from 
the ambulacrals. This may well be the case, and yet our knowledge of the constant 
changes in the growing Echinoderm skeleton, due to resorption and redeposition, renders 
in no way improbable the Suggestion that the auricles may have migrated from an 
interambulacral to an ambulacral position quite gradually. This appears to have been 
the opinion of M. Neumayr, (1889, «Stämme des Thierreiches» p. 370), who indeed 
spoke of this transition as the first stage in the development of the Glyphostomes. 
The a priori considerations already adduced suggest that it was really a conse¬ 
quence of other changes, and the facts of structure described in this memoir (espe- 
cially p. 116) show that it actually was not the first Step. In Mesodiadema and in 
Diademopsis incipiens , for instance, the internal processes were either still interam¬ 
bulacral or were reduced to an extended ridge; in Diademopsis Bowerbanlii this 
ridge remains, while at its adradial ends slight processes have arisen on the ambu¬ 
lacrals. Confirmatory evidence is desirable, but the facts seem to indicate that there 
really was a gradual movement of the attachments of the retractor muscles from 
an interambulacral to an ambulacral position. 
When once the ambulacrals had become concerned in building up the peri¬ 
gnathic girdle, a further check was placed on their free passage into the peristomial 
membrane. Thus the view of Loven, Neumayr, and Duncan that the peculiarities of 
the Diadematoid ambulacrum are due to pressure between the freshly formed plates 
