GENERA OF THE SUBORDER ORTHOIDEA 



59 



the muscular scars, dental lamellx and cardinal process 

 is essentially the same as in Group I Orth'ts [= Hes- 

 ffrorthis]." This statement has led to the popular 

 conception that Plt-ctorthh is a biconvex orthid with a 

 low cardinal area, having the internal structure of their 

 Orthis s. s. (Hrspfrorthis). Let us therefore examine 

 into the detailed internal anatomy of this genus, taking 

 up the various structures in the order named by Hall 

 and Clarke. 



The ventral muscle plan when examined in detail is 

 admittedly close to that seen in Orthis s. s. but is also 

 equally close to that of Hebertella. Resemblances to 

 Orthis s. s. are to be seen in the subcrescentic diductor 

 scars and the linear adductor track. However, this 

 difference is to be noted, that the diductor scars are 

 more expanded in front, exactly as those of Hebertella, 

 and the adductor track is double-ridged as in the latter 

 genus. 



Although the ventral musculature is similar to that 

 of Orthis s. s., the dental lamellae, cardinalia, and pallial 

 markings are so different as to set apart Plectorthis 

 from the Orthida;. The dental plates are thin septa 

 forming the walls of the delthyrial cavity. They also 

 bound deep lateral umbonal cavities. These lamellae 

 extend directly to the floor of the valve. The teeth 

 that they buttress are located at the angle between the 

 delthyrial and the hinge margin. In the Orthids the 

 teeth are usually situate laterally of the delthyrial mar- 

 gins. Plectorthis does not have the reniform ovarian 

 impressions characteristic of Orthis and lacks also the 

 elongate median ridge which divides the two pallial 

 trunks extending forward from the anterior ends of 

 the diductor scars. Plectorthis has a slight median 

 ridge extending forward from the adductor track but 

 it occupies a distance of one or two millimeters only. 

 Accordingly, the aggregate of ventral internal features 

 and the absence of certain pallial and ovarian markings 

 universal in the Orthida: relate the ventral shell of 

 Plectorthis to Hebertella rather than to Orthis s. s. 



When the structures of the dorsal valve of Plector- 

 this are studied, the true relationship of the genus to 

 Hebertella and to Platystrofhia becomes apparent. 

 The chief resemblance between these genera and Plect- 

 orthis lies in the character of the cardinalia and it is 

 here also that the most fundamental distinction between 

 Orthis s. s. and Plectorthis is to be found. The brach- 

 iophores of the latter are blunt plates intimately united 

 with thin, supporting plates which converge toward 

 each other and unite with the floor of the valve beneath 

 the cardinal process. This type of cardinalia occurs 

 also in the genera Mimella, Hebertella, and Doleroides, 

 all members of the Plectorthinae. Plectorthis is further 

 characterized by the presence of a small concave plate, 

 the fulcral plate, attached to the brachiophore and to 

 the inner wall of the valve. This plate defines the 

 rather deep socket and serves to strengthen the union 

 of the brachiophorcs and their supporting plates to the 

 walls of the valves. This structure is in marked con- 



trast to that of Orthis s. s. in wliich the brachiophorcs 

 are simple rodlike elements supported by the swelling 

 of adventitious shell deposited on the notothyrial plat- 

 form and walls of tlie valve. 



Hall and Clarke found further resemblance in the 

 cardinal processes of Plectorthis and Orthis s. s. In the 

 latter this process is alw.iys a simple ridge, not uncom- 

 monly bladelikc in appearance. It is difficult to find 

 scars of muscle attachment on such a ridge but they 

 have been seen on its sides and at its base. The car- 

 dinal process of Plectorthis, however, is like that of 

 Dinorthis in having a stout compressed shaft, which 

 bears a crcnulated myophore. The crenulation may 

 be on the posterior surface but is usually impressed on 

 the sides of the process, forming an excavation on each 

 side and thus narrowing the posterior part of the struc- 

 ture. This type of cardinal process also characterizes 

 Hebertella and makes the resemblance to that genus 

 all the more striking. 



Another minor point of difference between Orthis 

 s. s. and Plectorthis is to be found in the interlocking 

 of the valves along the front and lateral margins. In 

 the Orthidje as described on an earlier page the spaces 

 between the ribs on the outside correspond to double 

 ridges on the inside, but in Plectorthis the interspaces 

 form simple rounded costella: on the inside of the 

 front margin. 



In North America Plectorthis is known with cer- 

 tainty in the early Trenton limestone and ranges 

 through the Mohawkian into the Richmond of the 

 Upper Ordovician. Two species from the Chazy, 

 Hebertella exfoliata Raymond and Plectorthis holdeni 

 Willard, have been doubtfully referred to this genus, 

 but in neither form as yet has the interior been seen. 



Plectorthis is not known certainly in Europe. From 

 examination of the figures of several species referred to 

 Plectorthis in Cowper Reed's work on the Girvan dis- 

 trict, the writers would not admit a single one into the 

 genus as here defined. However, Hebertella scotica 

 (McCoy) has the external characteristics of Plectorthis 

 and Reed's figures of the dorsal interior show struc- 

 tures similar to those in this genus. The other species 

 referred to Plectorthis are to be distributed among the 

 genera Orthis s. s., Schizoramma, and Dolerorthis. 



Plectorthis did not survive into the Silurian. It may 

 have originated in the Chazyan, but more probably in 

 the Canadian, and apparently arose out of Finkeln-. 

 burgia, which has cardinalia that could readily have 

 evolved into those of Plectorthis (see p. 56). 



Genus HEBERTELLA Hall and Clarke 1892 



PI. 11, figs. 14, 17, 19-26 



Hall and Clarke, Pal. N. Y., vol. 8, pt. I, 1892, pp. 198, 

 222, pi. Sa, figs. 1-10. 



Genotype.— Or</)« sinuata Hall 1847, Pal. N. Y., 

 vol. 1, p. 128, pi. 32B, fig. 2. 



