belonging to the Class Palliobranchiata. 31 



the last character is however variable, even in the same species : 

 thus sonic varieties of Atrypa reticularis from the Eifel have it 

 apical and entire ; others, subapical and entire j others again have 

 it apical and emarginate ; and others, subapical and einarginate ; 

 even some varieties have a well-defined area. Generally however 

 the foramen is apical and emarginate, being notched inferiorly 

 by an open but concealed deltidium, as in Atrypa concentrica, A. 

 lioissyi, A. pectinifera, A. prunum, A. tumida, &c. 



In all the dentigerous Palliobranchs, a triangular space intervenes between 

 the condyles ; this it is proposed to term the ** deUid'mm :" when open, as in 

 certain Spirifers, &c, it may be termed an "open deltidium/' and when closed 

 or cicatrized, as often occurs, a " cicatrized deltidium :" it is " concealed" in 

 the A try pas generally, and in certain species of other genera, as Peniamerus 

 Knightii, &c. (in consequence of being occupied by the umbone of the ven- 

 tral valve), and "exposed" in the Spirifers, Martinias, Orthises, Leptsenas, 

 Strophomenas, &c, and in certain species of other genera, as Peniamerus 

 conchidium, Atrypa fait a, &c. The part forming a cicatrized deltidium may 

 be named the " cicatrix." The umbone of the Terebratnlas is furnished with 

 an aperture which is generally termed the "foramen;" it is " apical" when, 

 situated at the point of the beak (Terebratula dorsata, &c), and " sub- 

 apical " when placed below the point (Hypothyris) : often it is notched in- 

 feriorly by an open deltidium, as in Atrypa lioissyi, &c, in which case it is 

 an " emarginate foramen-" when not in this state it is an " entire foramen :" 

 in Terebratula dorsata it is " entire and apical" in Uypothyris obsoleta 

 " entire and subapical" in Atrypa Roissyi and Terebratula caput-serpentis 

 " emarginate and apical" and in Uypothyris psittacea " emarginate and sub- 

 apical." The distinction between a foramen and a deltidium is necessary, 

 as the former in all cases served as a passage for the pedicle ; but the latter, 

 as in some Spirifers, Hypothyris excavata, Peniamerus conchidium, &c. 

 (which have an open deltidium), only occasionally answered this purpose : 

 when it was necessary for a shell with a cicatrized deltidium to be attached 

 by means of a pedicle, the cicatrix was perforated as exampled in Spirifer 

 heteroclitus, Orthis adscendens, &c. : in LepUena, although the deltidium is 

 open, it could not serve as a passage for the pedicle in consequence of being 

 completely occupied by a prominency situated on the hinge of the ventral 

 valve. The condyle sockets are often bounded inwardly by a ridge or wall 

 (hence the name " socket-wall") which is occasionally prolonged into the 

 cavity of the shell under the form of a plate, as in Orthis eximia, &c. ; to 

 distinguish the two resulting plates from those of the dorsal valve, it will be 

 convenient to name them "socket-plates." Besides being occasionally pro- 

 longed, the socket-walls are generally expanded laterally under a lamellar 

 form; occasionally these lateral expansions remain separated (Terebratula 

 variabilis), but in general they are connate (Terebratula dorsata, Hypothyris 

 rostrata, &e.) and form a single plate; as this plate generally serves as the 

 base of the " crura of the loop " (Owen), it is proposed to term it the " crural 

 base :" it is " concave " in Terebratula dorsata, "flat" in Hypothyris ros- 

 trata, and " divided" in Terebratida variabilis. The crural base is often 

 supported by a plate extending along the medio-longitudinal line of the 

 shell ; the dorsal valve occasionally possesses a similarly situated plate : both 

 may be termed individually a " mesial plate." Care must be taken not to 

 confound the socket-plates with two ridges to be seen diverging from the 

 centre of the hinge and traversing the muscular impressions in certain shells 

 (Orthis Verneuili, &c.) : these appear to have been produced by two large 

 vessels belonging to the vascular system of the mollusk. 



