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



The spondylium is developed as the "platform" in Liiigulasmatidfe 

 aud TrimerellidiP of the Atremata; as a "spoudylium" in Pentamera- 

 cea of tlie Protreniata, and in Cyrtina, Caniei'ospira, Merista, and 

 Dicaiuara, of the Telotreinata. In the Atremata and Telotremata, 

 spondylia-bearing species are not numerous, but the individuals are 

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

 logic duration. 



The development of the sjjondylium or its morphologic equivalent 

 probably had its origin in an excessive deposit of testaceous matter 

 about the bases of the jDowerful adductors, diductors, and pedicle 

 muscles. GroAvth of the individual necessitates the progressive ante- 

 rior 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 i^ressure of the genitalia 

 against the anterior base of the forming spondylium, and since pres- 

 sure causes resorption or diverts testaceous dei)osition, it follows that 

 these organs will gradually produce cavities for their relief beneath 

 this plate. In the older species of the Trimerellidte and in all of the 

 Lingulasmatidie 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 spondylium is a thin, freely terminating or medially supported 

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

 is likewise thin and excavated in the order Telotremata. 



Hall and Clarke advance quite a different explanation as to the 

 origin of the spondylium. They write : ^ 



The spondylium is an area of muscular implantation. In its early or incipient con- 

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

 dental lamell?e, and forms a receptacle for the proximal portion of the pedicle, and 

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

 incipent condition, where, as in Orihis, it is represented only by the convergent den- 

 tal plates which usually iinite ^Yith, or rest upon the bottom of the valve, and 

 inclose only the base of the pedicle and its muscles, it will he 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 pedicle, whicli hats become involved or 

 inclosed by the growth of the pedicle-valve, and further modified by the develop- 

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

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

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

 ment in which 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 deltidinm. Thus the spondylium appears to be but the complement of the del- 

 tidium, or the original plate formed upon the body of the embryo, and that portion 

 of the adult shell to which the term deltidium has been applied, is the other part of 

 the original or primitive deltidial plate or pedicle-sheath. 



> Palseontology of New Tork, Vol. VIII, Part II, 1895, p. 332. 



