scHucHERT.] THE PRODELTIDIUM. 93 



the posterior margin of tbe same valve, and the pedicle is believed to 

 emerge between the i)late and tlie valve (Diseiiiisca). The i)rodelti(lium 

 is therefore alike in linal position in the jSTeotreinata and Protremata. 

 In the Atremata this plate is either attached by its entire surface or by 

 the posterior margin only to the dorsal sliell, as in Glottidia, where the 

 earliest embryonic stages are also niiknown. The prodeltidium is like- 

 wise dorsal in the cephalula stage of Thecidium (Protremata), but sub- 

 sequently is attached to the ventral shell, yet in reality remains dorsal 

 to the animal. In Glottidia (Atremata) this plate remains attached to 

 the dorsal valve, and in nowise aftects tlie pedicle opening, as in the 

 Keotremata and Protremata. In the Telotremata the prodeltidium has 

 not been observed, nor has any fossil species in this order shown the 

 least trace of a deltidium, and wherever the deltliyrium is closed it is 

 always by iilates growing medially from its walls, secreted by the 

 mantle and never by the peduncle. Therefore, when the prodeltidium 

 remains stationary or with the dorsal valve, it is not known that this 

 plate affects the original pedicle opening (Atremata and Telotremata), 

 but when subsequently attached to the ventral valve and partly sur- 

 rounds the pedicle with shell matter, it completely modifies the f>rimi- 

 tive pedicle oi)ening by restricting it to the ventral shell (Xeotremata 

 and Protremata). In the derived or later-appearing families of the Xeo- 

 tremata and Protremata the effects of foraminal modification initiated 

 by the prodeltidium may be wholly lost, as in Craniidje and Orthiida'. 



SIGNIFICANCE OF THE PRODELTIDIUM. 



The deltidium is the chief character of ordinal importance in the Pro- 

 tremata, and since this plate is attached to the ventral valve, ytt 

 originates in the dorsal prodeltidium, it seems reasonable to assume 

 that if similar developmental couditious are found in other orders such 

 orders would possess closer phylogeneticrelationshii) than those having 

 difieriug conditions. It has been shown that the prodeltidium is also 

 attached to the ventral valve in the Xeotremata, and so far both orders 

 show relationship in their earliest embryonic growth. Beecher has 

 shown that the protegulum or initial shell of the Protremata is discin- 

 oid in form and more like that of the Xeotremata than that of the 

 Atremata or Telotremata. He writes:^ 



Discinieca shows a subcircular ventral protegulum with a pedicle notch, and the 

 evidence of any hinge in the dorsal protegulum is very slight. The disciuoid char- 

 acter appearing in the second and third nepionic stage of the Paleozoic Orbiculoidea 

 has become so accelerated in Neozoic and recent Disciuisca as to produce a discinoid 

 protegulum. 



The strophomenoid shells usually retain a normal protegulum in the dorsal valve, 

 but from the acceleration of the discinoid stage in the ventral valve the protegulum, 

 has an abbreviate hinge and arcuate hinge line. (P. 346.) 



The nepionic stage of Leptwna rhomboidalis is represented by a shell without radii, 

 having a comparatively large i)edicle opening in the ventral valve and a large deltid- 

 ium. The hinge is not well defined and the shell is discinoid in form. * * » 



'Am. Jour. Sci., 3d series, Vol. XLI, 1891, p. 346 f Vol. XLIV, 1892, pp. 150-151. 



