scHuciiKiiT.i CHILIDIUM AND SPONDYLIUM. 99 



easily classified by the covered or open deltliyria, respectively. In some 

 of tlie Telotremata, toward maturity the deltidial plates aueliylose 

 medially posterior to the pedicle, or they may surround the pedicle, 

 thus resembling the deltidinm, but, since their origin is quite different, 

 they are termed "pseudodeltidia." Such pseudodeltidia in Cyrtia, 

 Oyrtiua, and some spirifers resemble the deltidium of Clitambonites. 

 Even the median line of anchylosis is often obliterated by the contin- 

 uous secretion of the completely united prolongations of the ventral 

 mantle lobe. In the rentamerida' the deltidium is generally absent, as 

 in the Orthid;c, but in Pentamerus and Conchidium it is often retained 

 as a thin, fragile, concave plate. This reversal in form from the gen- 

 erally prevalent, convex, or tiat deltidium may be due to the rostrate 

 and arched ventral umbones so common in these genera. . In the aber- 

 rant rostrate genus Dictyonella, which has an arched ventral umbone, 

 a concave plate is also present, between which and the shell the pedicle 

 passes and emerges upon the umbone, as in the Siphouotretidie. It is 

 uot certainly known that this jihite in Dictyonella is a deltidium, but its 

 form and position in the rostral cavity are very suggestive of that 

 organ in Pentamerus and Conchidium. The peculiar umbonal pedicle 

 opening in Dictyonella also finds its ecjuivalent in Leptaina. 



THE CHILIDIUM. 



The chilidium is a convex plate often covering the cardinal process 

 of the dorsal valve in the Protremata. It is particularly well devel- 

 oped in the families Clitambonitidie and Strophomenida\ and is not to 

 be confounded with the deltidium, since it first makes its appearance 

 not earlier than neanic growth, and apparently is a secretion of the 

 dorsal mantle lobe. The origin of the chilidium and of the deltidium 

 is therefore wholly different, and both have very dissimilar phyletic 

 significance. 



ORIGIN AND FUNCTION OF THE SPONDYLIUM. 



The spondylium is an internal ventral plate traversing the iiosterior 

 portion of the animal. The upper surface of this plate is usually trans- 

 versely marked by stri.e, which, in the Pentameracea have three distinct 

 curvatures in passing over it. 



Since their position and tlie area occu])io(l agree with the muscular scars of this 

 valve in Ortliis, they are hero regarded as homologous with the adductors, didnctors, 

 and adjusters of that genus. In Linr/ulasma, Liiu/ulopn aud the trimerellids the mus- 

 cular scars are not found in front nor underneath, but on the "platform"' of those 

 genera. The platform, therefore, is homologous with the spondylium of Clltumhon- 

 itcH and Pentamerus. * * * The portion of the valve immediately beneath the 

 spondylium, and occasionally the sides of the septum, are strongly marked by the 

 genital sinuses. Since there is no space posterior to these markings for the attach- 

 ment of the muscles, this clearly indicates that they were situated on the upper 

 surface of the spondylium.' 



1 "Winchell and Schuchert, Final Kept. Minn. Geol. Survey, Vol. HI, Part I, June, 1893, p. 378. 



