100 SYNOPSIS OF AMERICAN FOSSIL RRACHIOPODA. [bull. 87. 



The spondylium is developed as the " platform " in Liiigulasmatidte 

 and Ti'imerellida^ of the Atremata; as a "spondyliunr'' in Pentaniera- 

 cea of the Protremata, and in (Jyrtiua, Cameiospira, Meiista, and 

 Dicamara, of the Telotreniata. In the Atremata and Telotremata, 

 spondylia-bearing species are not nuinerons, bnt the individuals are 

 usually abundant, often of large size, and generally are of short geo- 

 logic duration. 



The development of the spondylium or its morphologic equivalent 

 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 progres.sive ante- 

 rior movement of the jnuscles, and when these are large tliere 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 pres- 

 sure causes resorption or diverts testaceous deposition, it follows tliat 

 these organs will gradually produce cavities for their relief beneath 

 this i^late. In the older species of the Trimerellid;e and in all of the 

 Lingulasmatidai displacement of the genitalia does not appear to have 

 been excessive, as the platforms are but slightly excavated. However, 

 in the terminal genus Trimerella the genitalia chambers are very deep, 

 and these are present in both valves. Throughout the Pentameracea 

 the sjiondylium is a thin, freely terminating or medially supported 

 plate, and never solid as in the older species of the Trimerellidic It 

 is likewise thin and excavated in the order Telotremata. 



Hall and Clarke advance quite a difterent explanation as to the 

 origin of the spondylium. Thej^ write : ^ 



Tbe spondylium is an area of mu.scular implantation. In its oarly or incipient con- 

 dition it is evident that it originates from tlie convergence and coalescence of the 

 dental lamellte, and forms a receptacle lor the proximal portion of the pedicle, and 

 for the capsular or pedicle muscles. * * * Considering this structure in its 

 incipent condition, where, as in Orthis, it is rei)resented only hy the convergent den- 

 tal plates Avhich usually unite with, or rest upon the bottom of the valve, and 

 inclose only the base of the pedicle and its muscles, it will be evident that the plate 

 is actually but a modification of the original pedicle-sheath. It is evidently the 

 inner moiety of this sheath surrounding the j)edicl(>, whicli has become involved or 

 inclosed by the growth of the pedicle-valve, and further modilied by the d(^veloi)- 

 mcnt of articulating processes where it comes in contact with the brachial valve 

 It therefore follows, as a natural infercuice, that wherever the spondylium is pres. 

 ent, whether in the incipient condition or in the more advanced stage of develo])- 

 ment in Avhich it supports all the muscles of the valve, it is, or, at some period of 

 growth, has been accompanied by the external portion of the sheath, which is termed 

 the deltidium. Thus the spondylium appears to be but the complement of the del- 

 tidium, or the original ])late formed upon the body of the embryo, and that portion 

 of the adult shell to which the term deltidium has been ap])liod, is the other part of 

 the original or primitive deltidial plate or pedicle-sheath. 



' Palteontology of New York, Vol. VUI, Part II, 1895, p. 332. 



