SCHUCBERT.l 



THE SPONDYLIUM. 101 



The writer also previously entertained this view, but wlieii it became 

 known that spoiidylia arc doveh)pe(l wliore no dental laiiiella' exist, as 

 in the Lingulasmatidie and TrimercllidiL' of the Atremata; that spoii- 

 dylia are never present in the Neotremata, where a pedicle- slieatli 

 is soiiietiines well developed, as in the Acrotretida'; and finally, that 

 a spondylium is oven present where no deltidiuin ever existed, as in 

 the two first-mentioned families, and in Cyrtina, Camerospira, jNlerista, 

 and Dicaniaia of the Telotreniata, such an cx])lanation became unten- 

 able. The fact that solid or excavated si)ondylia exist in three orders, 

 two of which never developed a ])edicle-sheath (Atremata and Telotre- 

 niata), and one had no dental lamella' (Atremata), is o()od evidence that 

 the prodeltidium i)riinarily had nothiiiii- to do with the develo])ment of 

 spoudylia. Further, no spondylia are developed in the Cambrian until 

 long after the deltidium was well established, and therefore the spon- 

 dylium can not be "but a modification of the original pedicle-sheath." 

 However, it is very probable that when the dental lamella' in the Pro- 

 tremata became sufliciently wide to join the ventral shell, crowding all 

 the muscles of this valve into a small area, these took advantage of the 

 inner sides of the dental lamella' for insertion, and thus a continuous 

 layer of testaceous matter was deposited within the rostral cavity. 

 With growth, the muscles move forward and press against the genitalia, 

 which causes resori)tion or nondeposition for their relief. Xo spondylia 

 appear before the Upper Cambrian, and here also are the first com- 

 pletely developed dental lamelhe. The so-called Lower Cambrian 

 camarellas have no comi:>letely developed dental lamelhe, and are 

 related to the rhynchonelloid genus Protorhyncha, and to l^riAorthis 

 bijlingsi, which also has no spondylium.' Therefore, the further conclu- 

 sion of Hall and Clarke can not be accepted, that, "where the teeth 

 are wholly without dental lamelhe, or where such lamelhe do not extend 

 to the bottom of the valve, it seems necessary to regard them as 

 instances of degeneracy or resorption of the primitive spondylium. "^ 



It seems clear to the writer that since the "shoe-lifter" plate, or 

 si)ondylium, in Merista and Dicamara is for muscular insertion, this 

 plate in the ventral valve of these genera is the morphic equivalent of 

 the spondylium in the Pentameracea, and that the dorsal muscular 

 plate in Dicamara is the equivalent of the cruraliuiu, ami can not "be 

 interpreted as an entirely different structure fr«mi the spondylium."'' 

 It is true that the spondylia of these genera are not exactly like those 

 of the Pentameracea, but since this jdate in the Atremata is not formed 

 by the union of dental lamelhe, as these do not exist in this order, there 

 is no reason for rejecting the terminology for these plates in Merista 

 and Dicamara. 



' Camarella minor and C. antiqua are more closely related to Protorhyncha than to any other genus. 

 Of Orthis biUingsi, the ty])c of Protorthis. very good casts of si)eciiiiens in Uie Cornell University 

 Museum are in the National Museum, which show that tliis genus also has no spondylium, and that its 

 characters are those of IJillingsella. 



'Hall and Clarke, ibid., p. 333. 



3 Ibid., p. 335. 



