MORPHOLOGY OF THE ORTHOID SHELL 23 



the deltidium is a normal and well developed structure, while in other species of the same genus it 

 is completely wanting. In the species without a deltidium, there is a well marked pedicle callist or 

 triangular scar which probably is the seat of pedicle attachment (see pi. 10, figs. 19, 22, 24). This 

 evidence appears to indicate that there is some connection between the deltidium and the pedicle. It 

 has been observed, further, that in orthid species having a deltidium, such as I lesperorthts tricen- 

 aria, Dinorthis szveetieyi, Valcourea, and others, the adductor and diductor muscles occupy the entire 

 delthyrial cavity, but in these forms there is no vestige of a pedicle attachment. Hence if there was 

 a functional pedicle, it must have been attached to some portion of the deltidium. These observations 

 were anticipated years ago by Winchell and Schuchert,* who clearly expressed the same views, as 

 follows: 



In all the species of Orth'u observed when a pedicle muscle [callist] is present a deltidium is absent; but 

 where this plate is developed the muscle [callist] is rudimentary. This evidence leads the writers to the conclu- 

 sion that the pedicle muscle is attached to the bottom of the valve in the apex of the delthyrium when the del- 

 tidium is wanting, but when it is developed the muscle is then more or less attached to the deltidium. 



In shells provided with a deltidium of the Deltatreta type, and having a prominent foramen, 

 it has been determined, at least for Deltatreta, that there is a well marked, even conspicuous pedicle 

 callist in the apex of the delthyrial chamber. Punctate shells are usually not provided with a del- 

 tidium, but when an apical plate is present, as in Pionodema, its position in the valve and the lack of 

 any visible scar of pedicle attachment on the floor of the delthyrial cavity make it clear that this plate 

 served for pedicle attachment. 



Lateral plates. — In many genera, especially those of the Orthidas, there are lateral plates 

 which restrict the delthyrium. Such are prominent in Glossorthis (pi. 4, fig. 28) and Ptycho- 

 pleurella (pi. 6, fig. 9). Perhaps the best example is Glossorthis, in one specimen of which the right 

 plate has been fractured slightly, emphasizing the suture line. In Hesferorthis the ventral shell has 

 not only these lateral plates but likewise a small deltidium, showing the close connection of these 

 different parts of the same structure secreted by the mantle. These lateral plates strongly resemble 

 deltidial plates in manner of growth and position. They apparently grow inward and forward by 

 marginal deposits of the mantle, since they show successive growth-lines as in deltidial plates. They 

 have, however, never been observed to meet and in this respect are not unlike incomplete deltidial 

 plates of certain of the Rhynchonellacea and Terebratulacea. These observations appear to us very 

 important as bearing on the source of deltidial plates in the order Telotremata. 



Less well developed plates of the sort described above occur also among the Pentameracea and 

 they have received considerable attention by other students of brachiopods. Hall and Clarke describe 

 them in several genera, and Kozlowski has gone so far as to place the Pentameracea in the Telotre- 

 mata because of their presence in that group. 



MODIFICATIONS OF THE NOTOTHYRIUM (CHILIDIUM 

 AND CHILIDIAL PLATES) 



In the dorsal valve the covering of the notothyrium is termed the chilidium. This structure is 

 usually absent in post-Cambrian orthids but occurs sporadically (probably as re-developments) or in 

 modified form in later genera. The usual type, seen in Vellamo, is a convex, perforate arch built 

 with its piers against the margin of the notothyrium and with the suture line of contact commonly 

 healed or grown over so that the interarea and chilidium appear continuous. The outer surface of 

 the arch is marked by convex lines or thin lamella; of growth. On the under side of the chilidium 

 the attachments to the sides of the notothyrium are strengthened by callus deposit laid over the points 

 of contact and along the walls of the notothyrial cavity. In many species of the Clitambonitidas 

 the posterior surface or edge of the cardinal process is attached to the under or anterior surface of the 

 chilidium. This type of notothyrial covering occurs throughout the Billingsellidas and Clitambom- 

 tidse. It is also present in some genera of the hesperorthid tribe. 



*GeoI. Minnesota, vol. 3, pt. 1, 1895, p. 422. 



