MORPHOLOGY OF THE ORTHOID SHELL 27 



greatest development, it bears all the muscles of the valve, the central adductor, and the lateral diductor scars being 

 often clearly defined, while the posterior portion of the plate is still reserved for the attachment of the pedicle, if 

 functional. In the pentamcroids the median septum of the pedicle-valve supporting the spondylium, is formed in 

 a similar manner by a continuation and coalescence of the dental plates, and wherever the median supporting septum 

 exists in this group, it will probably be found to have this composition. Median and lateral septa, however, in the 

 valves of the Brachiopoda, have a highly diverse origin in different cases. In most instances, except where bear- 

 ing spondylia, they are evidently of muscular origin and surfaces of muscular attachment, as shown in Spirifrrina. 

 [The spondylium is of early manifestation.] It appears in a highly developed state in conjunction with the 

 unmodified deltidium [delthyrium], first in Protorthis, of the Cambrian, then in PoLytoechla, Syntrofhia, Clitam- 

 honites and ScenUium, of the early [Ordovician] and later Silurian and of the Devonian. 



Schuchert" formerly held that the spondylium 



probably had its origin in an excessive deposit of testaceous matter about the bases of the powerful adductors, 

 diductors, and pedicle muscles. Growth of the individual necessitates the progressive anterior movement of the 

 muscles, and when these are large there is but little or no space left between or outside of them for the viscera and 

 genitalia, which are therefore crowded farther and farther anteriorly. This condition naturally produces constant 

 pressure of the genitalia against the anterior base of the forming spondylium, and since pressure causes resorption 

 or diverts testaceous deposition, it follows that these organs will gradually produce cavities for their relief beneath 

 this plate. 



This explanation certainly appears true for the platform in the atrematous Trimerellidas, but it 

 is now apparent that this is not the way the thin, plated, true spondylia of the Clitambonitidae and 

 Pentameridas were made. It is clear that what Schuchert was describing is the origin of what is 

 now called the sessile spondylium (spondylium discretum) so well developed in the Billingsellidas. 



Hall and Clarke give the correct function for the true spondylium when they say that it is "an 

 area of muscular implantation," originating from the convergence and coalescence of the dental 

 lamellae, and uniting either with a median septum or with the floor of the shell, but their further 

 statements about its making, in connection with the deltidium, a pedicle-sheath that had its first 

 stimulus of growth in the prodeltidium is, as we now see, wrong. What Schuchert and the older 

 students of brachiopods did not see is that the muscle-bearing platforms of the Trimerellidas, the 

 spondylia of Clitatnbonites and Pentamerus, and the cella of Merista are not homologous structures, 

 since, as we now know, all are of independent origin, arising in different ways though functioning 

 more or less alike. Sessile spondylia (spondylia discreta) are common in the Middle and Upper 

 Cambrian, but true spondylia are not present until late in Cambrian time and are chiefly character- 

 istic of the Ordovician and Silurian, while the cella type of muscle plate is of Middle and Upper 

 Ordovician origin, appearing first in Cyclospra and Dayia. 



What Schuchert in 1 897 regarded as the primitive attached spondylium (= sessile spondylium), 

 Walcott in 1912 called the pseudospondylium, and states that it occurs in PNisusia, "Billingsella" 

 Eoorthisy Finkelnburgia (free in front) and Huenella (free at the sides). The sessile or "pseudo- 

 spondylium" of Eoorthis 



appears in Orthis ... of the Ordovician and later faunas, probably as a reversion from a free spondylium [here 

 Walcott, as we now know, is clearly wrong]. On the line of descent to Protorthis the pseudospondylium becomes 

 a free spondylium and continues on through Syntrophia and Clarkella into the Ordovician and Silurian Pentameridas 

 and Clitambonitidx.'^ 



Walcott asked Ulrich to comment on the kinds of spondylia, and this he did as follows." The 

 term spondylium, he says, 



applies only to the t}'pical free or medially supported umbonal camera or spoon . . . and corresponds to a ventral 

 muscular area which is raised above the floor of the valve and formed by the convergence and union of the dental 

 plates . . . The manner in which the spondylium is att.iched to the bottom of the valve is so variable that the 

 feature does not seem to be of more than generic consequence. 



Ulrich then points out the great variability in a number of genera. 



" Bull. 87, U. S. Geol. Surv., 1897, pp. 99-101. 

 "Walcott, 1912, p. 307. 

 "In Walcott, 1912, p. 308. 



