Braocnropopa. 921 
retained in Caoneres, PropuoTELia, StRoPpHALosIA and AULOSTEGES, 
become wholly obliterated in the direct line of productoid 
development. In all these forms the “reniform impressi ns” 
retained on the inner surface of the brachial valve are evidence 
of fleshy brachia possessing a similar curvature to those of the 
STROPHOMENID 2. 
This group of genera has long been designated by the family 
name PropucrTip», introduced by Gray in 1840, though, in cor- 
relating the various divisions of Waacen’s group, ArpHANEROPEG- 
mata, there would be excellent reason for considering the chone- 
tids and productids components of a subfamily inferior in value 
to the STropHzomeNID» and quantivalent to the divisions Ortho- 
thetine, W aagen, 1884, and Rasinesquinine, Schuchert (emendatus), 
1893. 
Returning to the point of departure, we shall find that in the 
genus, Orruts, which in its broadest significance is tantamount to 
the family Orru1p», Woodward, 1852, since the elimination of 
several heterogenous branches, the deltidium was resorbed at an 
early stage of growth, leaving the delthyrium a wide, uncovered 
aperture during all the later stages of existence. The pedicle in 
this group of shells was undoubtedly large and vigorously func- 
tional throughout all mature conditions, as it is very rarely that 
any secretions of calcareous matter are found in the apex of the 
delthyrium, such as are frequently observed in mature and senile 
conditions of Srirtrer. The sharp delimitation of the pedicle- 
cavity containing all the muscular scars of the pedicle-valve, 
which occurs in the earlier forms (those of Oxruis in its restricted 
meaning, such as O. callactis, O. costalis, etc.) is maintained in all 
the numerous subdivisions of the genus, with the exception of 
Ruremometxa in which there is a great expansion of’ the muscu- 
lar scars, similar to that in the Srropzomenip# and to which 
reference has just been made. Otherwise the sessile condition of 
the spondylium is not modified throughout the entire history of 
this group. 
The elevation of the spondylioid plate, or the base of the 
pedicle-cavity, into a true spondylium is a phenomenon of equally 
early age to the two conditions already discussed. It appears in 
a highly-developed state in conjunction with the unmodified 
deltidium, first in Prororruts, of the Cambrian, then in Potyte- 
116 173 
