22 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



built with its piers against the angle formed by the palintrope and the dental plates. The piers are 

 usually buttressed against the track or axis formed by the growth of the tooth (see D, PI. A). The 

 attachment is strengthened by adventitious deposit laid over the whole inner surface and along the 

 sides of the dental plates and usually concealing all sutural contacts. Externally the interarea and 

 deltidium appear to be continuous and a suture is rarely visible unless the deltidium is somewhat 

 broken in. Shells having lateral plates in addition to the deltidium, however, commonly show 

 suture lines (see below) where the lateral plates overlap the deltidium. In the Clitambonitidas, 

 and in the older Billingsellidse as well, it is not uncommon for the deltidium to carry an axial stif- 

 fening along its inner surface. Hence the deltidium is a distinct plate, deposited in all probability 

 by the mantle, and is not a part of the palintrope or cardinal area. It is therefore not homologous 

 with the homoeodeltidium, as stated by Walcott and Schuchert — a change of opinion of very great 

 significance in classification. 



In Hesferorthis the deltidium, or "apical plate" as it has commonly been called, is truly an arch 

 as in Clkambonitesy and its piers are also built against the sides of the delthyrial cavity. Since 

 Hesferorthis commonly has in addition lateral plates or extensions of the palintrope over the margins 

 of the delthyrium, the deltidium rarely is arched above the interarea. On the other hand, the del- 

 tidium of Hesferorthis has been homologized with the apical plate of Sfirifer. In S. arenosus and 

 many other forms of the genus Sfirifer {sensu lato), the so called apical plate is a callosity filling 

 the back end of the notothyrial cavity but rarely if ever attaining the level of the interarea. It is not 

 a deltidium as in Hesferorthis, and accordingly the various structures are not homologous, although 

 they probably serve the same purpose, that of pedicle attachment. 



In a few of the punctate orthids or Dalmanellacea, apical plates have been noticed. They are 

 especially well developed in the Schizophoriidas and Parmorthis, but in no wise do they suggest a 

 deltidium as defined above. The structure is not an arch, but a flat plate flush with the interareas. 

 In Pionodema the anterior portion of the plate is concentrically lined and bevelled sharply below 

 the level of the interarea, suggesting that this small structure served the same purpose as the pedicle 

 callist. Mystrofhora areola also has a flat plate, but here it lies beneath the level of the interarea. 



The deltidium may or may not be perforate at the apex. In some species of Billingsella there 

 is a minute perforation barely large enough for the passage of a hair. The resemblance of this apical 

 foramen to that seen in the Strophomenacea has been urged as one link of kinship between the two 

 groups. There are, however, no other anatomical or stratigraphic grounds to support this supposi- 

 tion, but whenever this apical foramen occurs it is assumed either to have functioned for the protru- 

 sion of a pedicle, reduced in these forms to a mere thread, like the byssal threads in some lamelli- 

 branchs, or to have furnished passage for the anus. 



In one large group of shells, the Clitambonacea (pi. A, fig. 3), there is commonly a conspicu- 

 ous apical foramen which undoubtedly served for the passage of the pedicle. Distinct scars of pedi- 

 cle attachment have been observed in Deltatreta on the under surface of the deltidium and the floor 

 of the delthyrial cavity. In CUtambonites and Estlandia marginata, the foramen is sealed at matur- 

 ity by shell substance deposited within it (see pi. 8, fig. 8). This clearly means the loss of a func- 

 tional pedicle in later mature life. 



Function of the deltidium. — The function of the deltidium in all brachiopods is as yet not 

 clearly understood, since our studies do not embrace the Strophomenacea wherein this structure has 

 its best development. From the fact that the deltidium is present in the oldest and most primitive 

 forms, we must conclude, however, that it had some functional importance to the brachiopod possess- 

 ing it. In the geologically younger species, where the deltidium is almost uniformly absent, life 

 without the structure was obviously possible. At first it appears difiicult to understand the value of 

 a structure such as the well developed deltidia in the Strophomenacea, where, by their forward 

 growth, the pedicle opening will be constricted more and more, and in some forms no opening at 

 all will be left for the protrusion of the pedicle {Strofheodonta of the strophomenids) ; whereas 

 in other genera the deltidium has been resorbed or not allowed to grow by a thickened and therefore 

 more vital pedicle. This problem is of great interest, and may be solved when the great line of 

 deltidium-bearing shells, the Strophomenacea, are restudied from this point of view. 



Among the Orthacea we note, however, the interesting fact that in some species of Valcourea 



