THE BKACHIOPOD SHELL. 309 



CRURALIUM. 



In the dorsal valve an immature cruralium is seen for the first time in Nisusia festinafa 

 (PL C, fig. Ig at d, and fig. Ij at cr) which is without a cardinal process; this appears to be 

 the first appearance in the Lower Cambrian of a muscle-supporting projection in the dorsal 

 valve of an articulate brachiopod. In Billingsella a cardinal process is developed (PI. LXXXV, 

 figs, lu and Iv) on a low crural thickening. This process is present in Eoorihis and Orthis 

 and is prominent in DinortJiis, Hehertella, and most of the Orthidse, Strophomenidse, and Pro- 

 ductidse. 



A well-developed cruralium in the dorsal valve is first seen in SyntropTiia (S. rotundata, 



PI. CHI, fig. 4e) of the Upper Cambrian. It is well developed in Clarkella, Conchidium, and 



other genera of the Pentameracea. 



PLATFORM. 



Hall and Clarke [1892c, p. 46]'' state that— 



The specialized postmedian testaceous deposit which has been termed the platform by Messrs. Davidson and 

 King occurs among Brachiopoda only in the genera LaJchmina l=]Veobolus], Lingulops, Lingulasma, Dinobolus, 

 Trimerella, Monomerella, and Khinoholus; it constitutes the principal diagnostic character of the family Trimerellidse 

 of those authors, and in so far' as any of these genera possesses this feature, so far, at least, it is a trimerellid. In its 

 chronogenesis it is accompanied by certain peculiar phenomena. According to our present knowledge, its first appear- 

 ance is in the little Lakhmina \=Neoholus\ from the primordial "Obolus beds" in the Salt Range of India; but in 

 American faunas, where the development of the group is best exemplified, it is first met in Dinobolus (in external fea- 

 tures the most oboloid form of the group) in the later faunas of the Lower Silurian: Black River, Trenton, Galena. 

 Thereupon follow in the still later fauna of the Hudson group the more pronounced linguloid genera, Lingulops and 

 Lingulasma. Not, however, until the introduction of the Niagara or Wenlock fauna does the entire group, with the 

 exclusion of the inceptive linguloid forms, reach its culmination in specific and individual development. * * * 

 With the disappearance of this fauna the platform-bearing brachiopods virtually became extinct, and we have as yet 

 no trace whatever of the occurrence of this peculiar feature at any later date or in any other group of these animals. 



These authors conclude that it is probable — 



that the inception of the platform is due to the slight variation indicated in the mode or rate of formation of the muscular 

 fulcra in Lingula, and this may itself have been due in part to a simple increase in the size of the muscular bands. 



In summing up their conclusions on the source and development of the platform in the 

 trimerellids, the statement is made that "we are confronted with the interesting phenomena 

 of a similar resultant attained along difl^erent lines of development." This is expressed in a 

 diagram in which Oholella and Lingulella are taken as the first stages of two lines of develop- 

 ment. On the line of Obolella come in succession Obolus, Elkania, Dinobolus, and Tninerella. 

 On the line of Lingulella come in succession Lingula, Lingulops, Lingulasma, Trimerella, 

 and, as an offshoot from Lingulasma, Monomerella and Rliinobolus. Lalclimina [= Neobolus] 

 is placed in a central line between Obolus and Lingulella [Hall and Clarke, 1892c, p. 52]. For 

 the fuU discussion of the views of Hall and Clarke the student should consult the original 

 work [Hall and Clarke, 1892c, pp. 46-65]. 



In the course of my studies of the Cambrian brachiopods I came to the conclusion that 

 the platforms so far as developed were the result of shell secretion beneath and about the points 

 of attachment of the muscles, also beneath the visceral cavity. In the dorsal valve of Rus- 

 tella (PI. I, fig. le) from the Lower Cambrian, the most primitive type of brachiopod known 

 to me, the central and anterolateral muscle scars are slightly raised above the general level 

 of the interior of the valve ; many natural casts of the interior of the ventral valve fail to show 

 traces of the points of attachment of the muscles. 



I have assumed that Obolus was divergent from the same stock or radicle as Rustella, and 

 that Lingulella is an early divergent from Obolus. (See schematic diagram, p. 317.) None 

 of the Lower Cambrian species of Obolus or Lingulella Itnown to me has any unusual thickening 

 of the valves over the visceral area or about the muscle scars, but in the Middle and Upper 

 Cambrian it is not uncommon to find more or less indication of it. Obolus miclcwitzi (PL X, 

 figs. 1, la-k) of the Middle Cambrian is a striking example, and Obolus apoUinis (PL VII) of 



o Observations on the development and function of the platform in the inarticulate Brachiopoda. 



