26 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Thomas'' challenged the general conception that these plates gave support to the teeth. He says: 

 As to the function of these plates there appears to be greater reason for regarding them as supports of the areal 

 portion of the pedicle valve than as dental supports. It is certain . . . that they are not in all cases directly con- 

 nected with the teeth, while the connection in other instances need not necessarily imply the primary function of 

 dental support. The delthyrium is situated at the weakest part of the areal portion and the plates are developed 

 along its lateral margins and adjacent to the deltidium. This is just the position where one would expect a strength- 

 ening of the valve to take place. It seems preferable, therefore, to call such "dental" lamellae the "delthyrial 

 supporting-plates." 



It has been the experience of the writers that in the Orthacea these lamellse are invariably con- 

 nected with the teeth. Thomas' argument that these plates support the palintrope at its weakest 

 point is not conclusive, because there are many strophomenoid genera with long interareas that have 

 no dental supports reaching the floor of the valve, and among the orthids the oldest known genus, 

 Nisusia, is also without them. Furthermore, shells having broad overhanging palintropes, such as 

 Hesferorthis and Plectorthis, are commonly provided with thin, delicate, short dental plates of the 

 receding type, but shells with shorter palintropes and more ventricose valves have ponderous dental 

 plates, e. g., Orthis s. s. and Archworthis. Where these lamellae are present, they undoubtedly serve 

 the double function of supporting the teeth and the palintrope. 



Dental plates may be described as receding when they extend as ridges under the teeth and 

 the palintrope, and finally attain the floor of the valve near the apex only. Examples of this type 

 are common in American orthids such as Hesferorthis and Schizoramma. The dental plates are 

 called advancing when they slope forward and are continued around the lateral margins of the muscle 

 field as more or less well defined ridges, as in Schizofhoria. The term obsolete may be applied to 

 dental plates when the lateral umbonal cavities have been so filled by adventitious testaceous matter 

 as to obliterate them. This condition is common in old shells, so that the observer, if he does not 

 take the precaution to examine interiors of young specimens of the same species, may fall into the 

 error of describing the particular species or genus as possessing no dental plates. Such an error was 

 made by Hall and Clarke* in their definition of Orthorhynchula, which they say is without dental 

 plates. The young of that genus, however, are provided with well developed receding dental 

 lamellas. 



SPONDYLIUM 



History of the term. — The term spondylium (from the Greek word for vertebra) was pro- 

 posed by Hall and Clarke (1892) in their discussion of Clitambonites. They say of Pronites 

 adscendens Pander:® 



On the interior of the valve the dental lameUae are very strongly developed, converging and uniting in the 

 median line before reaching the bottom of the valve; thus forming a spondylium [a spoon-shaped plate], which 

 with the deltidium encloses a conical subrostral vault . . . This term will apply with equal propriety to the similar 

 plate existing in the pedicle-valve of other brachiopods, e. g. . . . PentameruSy Camarella [Camarophona, etc.]. 



Later these authors add:^" 



It has become evident since the introduction of the term that these processes in the two valves [spondylium 

 and cruralium of Pentamerus] , though similar in aspect, are similar neither in origin nor function, and it becomes 

 necessary to modify the application of this term. Hence it is proposed to restrict the term spondylium to the plate 

 existing on the pedicle- valve, and to the plates of the brachial valve, whether united or discrete, the name 

 cruralium will be applied. . 



The spondylium is an area of muscular implantation. In its early or incipient condition it is evident that it 

 originates from the convergence and coalescence of the dental lamellae, and forms a receptacle for the proximal 

 portion of the pedicle, and for the . . . pedicle muscles. In Clitambonites and Pentamerus, where it attains its 



'Mem. Geol. Surv. Gt. Brit., Pal., vol. 1, pt. 2, 1910, pp. 100-101. 



«PaI. N. Y., vol. 8, pt. 2, 1893, p. 181. 



•Op. cit., pt. 1, p. 234. 



"Pt. 2, pp. 331-332, 335 footnote, 341. 



