306 CAMBRIAN BRACHIOPODA. 



connection with the notes on the cardinal areas. The evohition in the Atremata, Neotremata, 

 and Protremata has been in the same direction, ranging from the open-fissured protegulum 

 stage in pre-Cambrian time to the fully developed, clearly defiined, subtriangular, usually 

 convex pseudodelthyrium and deltidium. In Acrotreta Tcutorgai the cardinal area and deltid- 

 ium of the dorsal valve are well defined (PI. LXV, figs. 3i and 3j). I am inclined to think 

 that the pseudodeltidia of the Atremata and Neotremata, as also the deltidia of the Protre- 

 mata of the Cambrian, are all the result of holoperipheral growth and not of growth from a 

 prodeltidial plate, as interpreted by Beecher [1892, p. 142]. 



The deltidia of the Protremata vary in size from the large deltidium of Billingsella colora- 

 doensis (PI. LXXXV, figs. In and lo) to those in wMcli the delthyrium is entirely open in the 

 mature stages of the individual. The exact stage at wliich the rudimentary deltidia resulting 

 from the holoperipheral growth of the shell begin to take on the more definite form shown in 

 Nisusia (PL C) and Billingsella (PI. LXXXV) is unknown. 



From my studies of the Cambrian bracMopods I see notlfing that leads me to the behef 

 that in the Protremata the deltidium has a different origin from the pseudodeltidium of the 

 Atremata. Whatever the significance of the prodeltidium or tliird shell plate of the Neotremata 

 may be, it is plain from the many species of Atremata, Neotremata, and Protremata through- 

 out the Cambrian that the pedicle openings tend to develop in two directions; resulting, first, 

 in a more or less large open delthyrium, due in all probability to a tluck and short peduncle 

 filling up the space, and, second, in a more or less covered delthyrium (the covering being 

 either the phosphatic pseudodeltidium or the calcareous deltidium), the peduncular opening 

 being either between the two valves or reduced to a foramen limited to the ventral valve. 



LISTRIUM. 



Hall and Clarke [1892a, p. 153] deemed it important to apply the term listrium to the 

 calcareous deposition or plate closing the progressive track of the pedicle opening or pedicle 

 cleft posterior to the apex of the ventral valve. This feature is known only in Orbiculoidea 

 among Cambrian genera of BracMopoda. It has not been observed in the Siphonotretidse. 



PSEUDOCHILIDIUM. 



The pseudoclnlidium is a convex plate closing the primitive aperture in the dorsal valve 

 of Atremata and Protremata. It corresponds to the pseudodeltidium of the ventral valve and 

 is formed about the dorsal side of the pedicle aperture. 



CARDINAL PROCESS. 



The cardinal process, like the crura, was evidently developed in the interval of progression 

 represented between the progenitor of Kutorgina and Billingsella. The process is unknown 

 in Kutorgina, but is very clearly developed in the Middle Cambrian forms of Billingsella 

 (PI. LXXXV, figs, lu-y; PI. LXXXVI, figs. 31 and 3n; and PL LXXXVIII, figs. If and li). 

 I have not observed it in Nisusia or Nisusia (Jamesella) of the Lower and Middle Cambrian. 

 It appears in Eoorthis remnicha (PL XCI, figs. Iq and Ir; PL XCII, fig. 2b) of the Middle Cam- 

 brian, but I have not seen it distinctly in the Ordovician ProtortJiis (PL XCIX). 



The development of the process followed the definite cardinal area and hinge teeth and 

 must at first have been quite rudimentary. Its absence in Nisusia and its presence in Billings- 

 ella indicate that the latter is a more progressive type in this respect. In the Middle Ordo- 

 vician orthoid Dinortliis subquadrata the cardinal process is striated and fluted so as to give a 

 better holding surface for the attachment of the muscles. In the Silurian genera Waldheim^ia 

 and Cliothyris the entire concave part of the area of the dorsal valve projects as a plate poste- 

 riorly beyond the beak. 



The cardinal process becomes more specialized in its bifurcated condition in the Stropho- 

 menidse and Productidse, etc. (See Hall and Clarke [1892a, p. 168] for further description of 

 this process.) 



