scHucHEET.] CHILIDIUM AND SPONDYLIUM. 99 



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

 of the Telotremata, toward maturity the deltidial plates anchylose 

 medially i^osterior to the j)edicle, or they may surround the pedicle, 

 thus resembliug' the deltidium, but, siuce their origin is quite different, 

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

 Oyrtiua, and some spirifers resemble the deltidium of Olitambonites. 

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

 uous secretion of the completely uuited prolongations of the veutral 

 mantle lobe. In the Pentamerida' the deltidium is geuerally absent, as 

 in the OrtbidiC, 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 flat 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 i)late is also present, between which and the sliell the pedicle 

 l)asses and emerges u^ion the umbone, as in the Siphonotrctid;e. It is 

 not certainly known that this plate 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 

 oi)ening in Dictyonella also finds its equivalent in Lepttena. 



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 Clitambonitidai 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 jihyletic 

 significance. 



ORIGIN AND FUNCTION OF THE SPONDYLIUM. 



The spondylium is an internal ventral plate traversing the posterior 

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

 versely marked by strite, which, in the Peutameracea have three distinct 

 curvatures in passing over it. 



Since their position and tlie area occu]iied agree Tvitli the muscuhir scars of this 

 valve in Orthis, they are here regarded as houiologous with the adductors, diductors, 

 and adjusters of that genus. In LingnJasma, Lbujuiops and the trLuierellids the mus- 

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

 genera. The jjlatform, therefore, is homologous with the spondylium of ClUamhon- 

 ites 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 ujiper 

 surface of the spondylium.' 



'"Wincliell and Schuchert, Final Kept. Minn. Geol. Survey, A'ol. Ill, Part I, June, 189i!, p. 378. 



