36 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



platform as successive depositions of adventitious substance are laid upon it. The plan of the myophore 

 may become distinctly oval or even subcircular, the central ridge becoming more elevated especially 

 at the antero-ventral extremity. Such modifications result in a cardinal process not unlike that found 

 in some of the Dalmanellidas. In Dinorthis, wherein the two crenulated halves are divided by a 

 depression, the ventral portion of the cardinal process may become distinctly bilobed, the lobation 

 being produced by posterior growth of the cardinal process through the gradual enlargement of the 

 lobes with increasing age. The slight depression dividing the crenulated halves of the myophore is 

 not obliterated during growth. 



(4) The Hebertella type, which is not unlike the preceding in its initial stages, having a short 

 shaft confined within the notothyrial cavity and having the muscle attachment on its dorsal extremity. 

 It differs, however, in having the myophore in late stages impressed into the sides of the shaft. In 

 very youthful forms the muscles may be borne on the dorsal surface of the shaft as in the Dinor- 

 thidas, but with increasing age the muscle attachments are sunk below the surface of the sides of 

 the shaft. In old forms the myophore may appear as a rather slender septum crenulated on its sides 

 and mounting a more or less stout shaft. In gerontic forms the muscle attachments may be sunk 

 into the shaft at the dorsal portion of the cardinal process, but the ventral portion may be expanded 

 and bulbous. This type has been observed in Plectorthis, Doleroides, and other members of the 

 Plectorthidse (see pi. 11, figs. 4, 17, 23, 26). It has not, however, been definitely noted in Platy- 

 strofhia, in which the cardinal process is not uncommonly considerably resorbed and in some instances 

 reduced to a mere remnant. 



(5) The Dalmanella type, common to many endopunctate shells. Primitively the cardinal 

 process has a short shaft and a distinctly lobate myophore. The latter may be either bilobate as in 

 Heterorthina (pi. 17, fig. 32) or trilobate. The cardinal process of these shells is distinctly progres- 

 sive, since all the diductor attachments are on the myophore. Usually there are clearly visible four 

 distinct scars, but it is not uncommon for the middle two to be coalesced and in some instances 

 elevated above (in a posterior direction) the outer two, and in that event the myophore is distinctly 

 trilobate. In other forms the inner and outer set are combined, making a bilobed myophore. This 

 bilobation may be expressed in the shaft, and with progressive posterior growth of the myophore 

 there is left anteriorly a depression or cleft in the shaft (see pi. 1 7, fig. 32). As far as our researches 

 have gone, these variations of the posterior portion of the cardinal process have no generic value, nor 

 are they always constant in the same species. 



Old-age thickening of the cardinalia, and of the cardinal process in particular, produces riotous 

 growths. Such are seen commonly in Dalmanella meeki and another type occurs in Levenea sub- 

 carinata and Isorthis perelegans. In D. meeki and other dalmanellids the myophore may become so 

 enlarged as to fill the notothyrium completely. In Levenea and Isorthis the anterior portion of the 

 ventral lobe of the myophore may be produced forward along the shaft as a distinct process, giving to 

 the whole structure the appearance of a fly's head with "tongue" or proboscis protruded. 



There are several modifications of the Dalmanella type of cardinal process which should 

 be mentioned, since they are distinct enough to mark groups of shells. These are: 



(6) The Rhipidomella type, in which the shaft has its greatest extension in a ventral direction 

 (at right angles to the plane of the commissure) and is scarcely developed anteriorly. The myophore 

 is commonly large. The greatest development of this type of cardinal process is found in Platyorthis 

 planoconvexa, in which it is remarkably robust (see pi. 19, fig. 24). 



(7) The Parm^orthis visbyensis type, in which the myophore is like that of Dalmanella but has 

 been revolved 90° so that it is parallel to the plane of the commissure and is distinctly visible in 

 dorsal view. The shaft is essentially obsolete, as the brachiophore plates and the cardinal process 

 have become fused. This modification is no doubt developed to allow a greater pull of the muscles 

 on the myophore (see pi. 21, fig. 8). 



(8) The Heterorthis type (pi. 20, fig. 20), in which the median lobe of the myophore is elon- 

 gated posteriorly at the expense of the lateral lobes, producing thereby a carinate ridgelike structure 

 on the cardinal process. In molds of the interior this type appears to be a simple ridgelike cardinal 

 process, but in well preserved specimens the primitive lobation is apparent. 



