GENERA OF THE SUBORDER ORTHOIDEA 



111 



sloping slightly vcntro-latcrally and formed by the 

 inside surface of the teeth as they grow forward. It is 

 to this surface that the deltidium is attached. 



The upper surface of the spondylium is marked by 

 concentric lines which represent periods of no growth. 

 Superimposed upon these are longitudinal lines which 

 are taken to represent traces of muscle markings. No 

 signs of muscle attachment occur anywhere in the valve 

 except on the inner surface of the spondylium, making 

 it evident therefore that the function of this structure 

 was one of muscle anchorage. However, the indi- 

 vidual muscle marks have not been ascertained. In 

 one specimen were seen four divergent longitudinal 

 ridges on the upper surface of the spondylium. The 

 strongest two are at the bottom, with the other ones 

 some distance on the sides. 



It is deduced from the above that the adductor 

 muscles occupied the central part of the spondylium 

 over the median septum, and the diductors and adjus- 

 ters were located laterally on the sides of the spondy- 

 lium. The scar of attachment of the pedicle was 

 probably located at the rear. In Hemipronites a slight 

 callosity at the apex indicates the scar of pedicle attach- 

 ment. Deltatreta has a prominent pedicle callist at 

 the apex. 



The septum consists of a single piece (euseptum) 

 and is usually prolonged to the middle of the valve 

 or beyond. To this ensemble, consisting of a spoon- 

 shaped plate and euseptum, Kozlowski*" has applied 

 the name spondylium simplex. 



The deltidium of the Clitambonitids is always a 

 convex plate covering the delthyrium. Its sides are 

 buttressed against the upper surface of the ridge formed 

 by the growth of the teeth and are strengthened further 

 by deposits of callus on the sides of the teeth (pi. 7, 

 fig. 14), and in some forms on the walls of the spon- 

 dylium. The upper layer of shell on the interarea 

 appears to be continuous with the outer surface of the 

 deltidium. 



The deltidium in these forms is usually perforate 

 and far less commonly imperforate. Usually there is 

 a large foramen located near the apex, which suggests 

 that this opening functioned, at least during life, as the 

 pedicle opening. However, in some genera and species 

 the apical foramen was sealed at or before maturity, 

 thus doing away with this structural feature (see pi. 8, 

 fig. 8). The apical foramen is especially well devel- 

 oped in VellamOy and here appears to have persisted 

 throughout life, at least in the Trenton forms. Many 

 of the specimens from Minnesota have an elevated 

 flange or rim about the foramen and from its position 

 at the apex it would appear that the deltidium is formed 

 around the pedicle by the mantle. 



In the ventral valve the ovarian and pallial impres- 

 sions, where visible, are similar to those of Orth'ts and 

 Hesperorthis. As shown in Hemipronites, a pallial 



" Pal. Polonica, vol. I, 1929, p. 124. 



sinus extends from the outside extremities of the spon- 

 dylium in the position of the diductor tracks, along 

 each side of the median ridge, bending abruptly later- 

 ally at the front (see pi. 8, fig. 10). These sinuses 

 bound subreniform ovarian areas marked by elevated 

 wavy ridges exactly as in the Orthidx. In some 

 genera these markings arc much less distinct but in 

 all they strongly suggest direct affiliation with the 

 Orthidae. 



Dorsal valve. — The pattern of the dorsal valve in 

 most of the genera is alike. The brachiophores are 

 flattened or rodlike divergent plates forming the mar- 

 gins of the notothyrium and extending laterally into 

 the visceral cavity of the valve. In adults the noto- 

 thyrial platform is greatly thickened by adventitious 

 shell which extends laterally onto the inner surfaces 

 of the brachiophores and around their dorsal surface. 

 This gives the brachiophores the appearance of being 

 laterally prolonged plates when actually they are short 

 and hidden within the extra testaceous matter. The 

 postero-lateral face of the brachiophore, however, 

 always forms the inner wall of the socket, while on 

 the outer wall a denticle can usually be seen which 

 articulates with the socket in the postero-lateral face 

 of the tooth. 



The chilidium is formed similarly to the deltidium. 

 It is a convexly arched plate built against the sides of 

 the notothyrium and cemented by deposition of adven- 

 titious shell under the brachiophores and under the 

 anterior side of the chilidium. Not infrequently adven- 

 titious deposit is laid on the ventral surface of the 

 brachiophore and continuously with the front margin 

 of the chilidium. The precise function of the chilidium 

 is, at present, not known, but it may be of use to the 

 animal in protecting the muscle attachments on the 

 myophore of the cardinal process. 



The musculature of the dorsal valve is like that seen 

 in Orthis, Hesperorthis, and other orthids. There are 

 four adductor scars divided centrally by a stout median 

 ridge and horizontally by ridges at right angles to the 

 median elevation. The median ridge, its lateral ridge, 

 and the continuous curved ridges of the chilidium and 

 lateral thickenings under the anterior face of the brach- 

 iophores give the appearance of an anchor when the 

 shell is inverted with the posterior toward the observer. 

 This analogy was emphasized by Pahlen*^ in his study 

 of the genus Orthisina. The anterior pair of adduc- 

 tors are not uncommonly divisible into two separate 

 scars, as in Productorthis. 



The cardinal process is simple, orthoid in structure, 

 and may be continued posteriorly so as to unite with 

 the inner surface of the chilidium. Pallial markings are 

 usually not clearly visible in the dorsal valve. Most 

 shells show two oblique ridges extending antero-later- 

 ally from the adductor pits to a little beyond the 

 anterior margins of the muscle field. 



" Mem. .Acad. Imp. Sci. St. Petersb., 7th ser., vol. 24, 

 no. 8, 1877, p. 7. 



