MORPHOLOGY OF THE ORTHOID SHELL 39 



face of the brachiophore, fulcral plates being absent. The brachial apparatus is supported by adven- 

 titious shell substance only, deposited on the inside surface of the brachiophores and under the antero- 

 dorsal edge. This gives, in old shells, a cuplike appearance. This type of brachiophore is best seen 

 in Leveuea, Dalmanella, Cannijerella, etc. 



(8) Parniorl/iis type (see pi. 21, figs. 3, 13). — The brachiophores here are supported by short 

 divergent plates and the socket is defined by a concave fulcral plate. This type is found in Parmorthis, 

 Mendacella, etc. 



(9) Watisella type (see pi. 22, figs. 9, 22, 23, 25, 29). — Like the preceding except that the 

 brachiophore plates are subparallel or convergent and unite with the median ridge. Fulcral plates 

 are present. 



(10) Heterorthts type (see pi. 20, figs. 19, 20). — In this group the brachiophores are dalman- 

 ellid-like, but supported by a deposit of adventitious substance speading from the notothyrial 

 platform and the floor of the valve under the dorsal edge of the brachiophore. This is clearly a 

 modification of the Dahnanella type and is seen also in Smeathenella^ Reuschella, and Harknessella. 

 In Rhipidomella and its allies, support of the brachiophores is by means of adventitious substance from 

 the floor of the valve, which unites with the dorsal edge of the brachiophore; here, too, there also is 

 not uncommonly a rather long sharp crural process extending from the brachiophore. 



CRURALIUM AND PSEUDOCRURALIUM 



A cruralium is not often seen in the Orthoidea, but some stocks have developed it, as the Skeni- 

 diidas, Mystrophoridse, and Linoporellidse. Some of the Plectorthida; have a sessile cruralium. In 

 the Billingsellidas and Orthidse a pseudocruralium is not uncommon; it is the same structure that we 

 have called notothyrial platform. 



SEPTA 



The dorsal valve of the Orthoidea is almost never without some sort of axial thickening to 

 strengthen the valve between the adductor muscles. This thickening is usually in the form of a low 

 broad ridge, as in Hesperorthis (see pi. 4, fig. 17), and rarely takes the form of a narrow septum. 

 In a few families and genera the median septum forms a prominent partition which divides the 

 valve into equal halves. Such is the case in the Skenidiidse, and also in Mystrophora, where the 

 septum is so high as nearly to reach the inner surface of the ventral valve. 



The median ridge of most orthids is clearly the homologue of the median septum of the 

 Rhynchonellacea, and in the Paleozoic these shells are scarcely ever without an axial thickening. 

 Aberrant genera such as Porambonites and Lycophoria are unique in not possessing a median septum. 



MUSCULATURE 



The individualization of the musculature of the dorsal valve is fraught with the same difficul- 

 ties as that of the ventral valve. In well preserved specimens there are usually the distinct scars of 

 eight muscles, but in a few species two additional scars are distinguishable. Most prominent and 

 usually best preserv^ed are the four (in many genera six) adductor scars produced by the bifurcation 

 of the two ventral adductor muscles as they pass from the ventral to the dorsal side. These form a 

 field of variable outline usually located near the center of the valve and anterior to the notothyrial 

 platform. These two pairs are almost invariably divided longitudinally by the median ridge and 

 are usually separated transversely by smaller, less prominent ridges at a greater or lesser angle to the 

 median one, thereby dividing the field into quadrants. The pair immediately in front of the noto- 

 thyrial platform are the posterior adductors, and the pair farther forward arc the anterior adductors. 



In many genera the anterior adductor impressions are clearly divisible each into two parts (nota- 

 bly true in Prodtu tort his y see pi. 3, fig. 1 1 ), so that there are evidently six scars in the adductor field. 

 This third "adductor" set may represent adjustor muscles whose ventral attachments are on the 

 pedicle and are thus not recorded by a corresponding set of impressions on the ventral valve. It is 

 possible also that the adductor muscles trifurcate in their passage from the ventral to the dorsal valve. 



Derby** early directed attention to the presence of six adductor scars in the dorsal valve of 



"BuU. CorneU Univ., vol. 1, pt. 2, 1874, p. 30. 



