SGHUCHERT.] THE PRODELTIDIUM. 93 



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

 emerge between tbe plate and the valve (Disciuisca). The prodeltidinm 

 is therefore alike in final position in the Neotremata and Protremata. 

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

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

 earliest embryonic stages are also unknown. The prodeltidinm 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 i)late remains attached to 

 the dorsal valve, and in nowise affects the pedicle opening, as in the 

 Neotremata and Protremata. In the Telotremata the prodeltidinm has 

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

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

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

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

 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 i^artly sur- 

 rounds the pedicle with shell matter, it comj)letely modifies the primi- 

 tive pedicle opening by restricting it to the ventral shell (Xeotremata 

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

 tremata and Protremata the effects of foraminal modification initiated 

 by the prodeltidinm may be wdiolly lost, as in Craniidae and Orthiidfe. 



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, yet 

 originates in the dorsal prodeltidinm, it seems reasonable to assume 

 that if similar developmental conditions are found in other orders such 

 orders would possess closer phylogenetic relationship than those having 

 differing conditions. It has been shown that the prodeltidinm is also 

 attached to the ventral valve in the JSTeotremata, 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 ]S"eotremata than that of the 

 Atremata or Telotremata. He writes: ^ 



Disciuisca 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 Lepta>na rhomboidaUs is represented by a shell without radii, 

 having a comparatively large pedicle 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. Soi., 3(1 series, Vol. XLI, 189t, p. 346; Vol. XLIV, ] 892, pp. 150-151. 



