126 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



under discussion, this secondary shell is deposited mainly 

 between the brachiophore plates in connection with the 

 cardinal process, but in Orthis testudinarta Dalman 

 from Borenshult, Sweden, the brachiophores are 

 swollen on the inside at their junction with the median 

 ridge and a very noticeable amount has also been 

 deposited in the cavities between the brachiophore and 

 the fulcral plates. If this deposition were carried to 

 its extreme, the fulcral plates would be obliterated and 

 the cardinalia would simulate those of Dahnanella, 

 Levenea, and other genera of the Dalmanellidas. 



In the ventral valve the differences are not so strik- 

 ing. However, the diductor scars are not so long and 

 flexuous, and the muscle impressions as a whole are 

 more heart-shaped than in Dalmanella. 



Wattsella is abundant in the Upper Ordovician of 

 England and Wales and in the early Silurian of 

 Sweden in strata previously regarded as Ordovician. 

 In the United States, it is known for the first time in 

 the early Silurian (Edgewood limestone of the Alexan- 

 drian series) of Illinois and Missouri. This is a strik- 

 ing fact which lends strength to the assertion made in 

 the discussion of Dalmanellay namely, that shells having 

 the structure of Orthis testudinarta Dalman are as yet 

 unknown in the North American Ordovician. In the 

 Silurian of Arisaig some of the specimens referred by 

 McLearn*' to Dalmanella elegantula actually belong 

 to Wattsella. It is not improbable that some of the 

 Silurian dalmanellids from Anticosti referred to D. 

 testudinarta by Twenhofel will prove to be Wattsellas 

 when the interiors are studied. Further, many of the 

 "Dalmanellas" collected by Williams in the Silurian of 

 Maine prove to belong to Bancroft's genus. 



Genus RESSERELLA Bancroft 1928 



PI. 17, figs. 14-16 



Bancroft, Mem. Proc. Manchester Lit. Philos. See, vol. 72, 

 1928, p. 54. 



Genolectotype. — Orthis canalis Sowerby 1839, 

 in Murchison, Sil. Syst., pi. 13, fig. 12a. 



Description. — Externally, Resserella (as here 

 restricted) forms a very precise homoeomorph of Dal- 

 manella as described in this memoir. The valves are 

 unequally biconvex, the dorsal being flatly convex and 

 the ventral rather strongly so. There is a shallow 

 sulcus on the dorsal valve that is defined from the 

 vicinity of the beak to the anterior margin. The beaks 

 and the interareas have the same features exhibited by 

 Dalmanella s. s. The external surface is multicostel- 

 late as in the American shells. 



Inside the ventral valve the identity with Dal- 

 manella is carried out to the form and lobation of the 

 muscle area. In the dorsal valve the cardinalia of 

 mature shells are identical with mature Dalmanella. 



"Geol. Surv. Canada, Mem. 137, 1924, pp. 53-54. 



In Resserella the adductor impressions of the dorsal 

 valve are subequal and form a rather circular pattern. 

 Discussion. — From the above i^ will be seen that 

 the interior and exterior of Resserella and Dalmanella 

 are very close if not identical in mature shells. For 

 this reason we had regarded Resserella as a synonym 

 of Dahnanella (restricted) in the early phases of our 

 work. However, in correspondence with Mr. Ban- 

 croft, he has assured us that there is a very close 

 relationship between Resserella and Wattsella, to be 

 seen in the presence of fulcral plates in the early 

 growth stages of the former. If these fulcral plates 

 exist in Resserella, we admit the validity of the genus. 

 We have not seen such plates in the material available 

 to us and must accept Bancroft's statement. 



The recognition of Resserella as a distinct genus 

 does not invalidate our genus Parmorthis, based on 

 Dalmanella elegantula, which Bancroft places in his 

 Resserella. The type selected by Bancroft for his 

 genus is Orthis canalis of Sowerby. As originally 

 described, this species was composite, including within 

 it Ordovician and Silurian specimens. On the basis of 

 the latter, Davidson made Sowerby 's species a synonym 

 of Orthis elegantula Dalman. The specimen from 

 the Ordovician, however, according to Sowerby's fig- 

 ures and remarks (pp. 630, 640), is different from the 

 Silurian forms, which clearly belong to "Dalmanella" 

 elegantula (Dalman) and were so referred by David- 

 son. On the other hand, specimens sent us by Mr. 

 Bancroft prove to be of the larger Ordovician form, 

 and it is on this shell and not on the "elegantula" type 

 that Resserella is based. 



Resserella as thus restricted consists of shells that are 

 more circular as a rule than those of Parmorthis. Fur- 

 thermore, the species of the latter genus have a rather 

 definite ribbing system unknown in Resserella, con- 

 sisting of a rather prominent median fascicle in both 

 valves that is so well developed in some of the species 

 as to produce a flattened or smooth portion in the 

 middle of the valves at the front. On the interior, 

 however, the diiTerences between Resserella and Parm- 

 orthis are still more pronounced. In the ventral 

 valve the strong arching of the beak produces a very 

 deep delthyrial cavity and strong dental plates, much 

 thicker than those of Resserella. Moreover, the ven- 

 tral muscle field of Parmorthis is not decidedly lobate 

 as is the corresponding area in Resserella. Inside the 

 dorsal valve the differences are equally striking. In 

 Resserella as known to us there is no prominent thick- 

 ening produced laterally along the periphery of the 

 adductor field, whereas this same area in Parmorthis 

 has a very distinctive shape shared only by the closely 

 related Fascicostella. Here the adductor area is elon- 

 gate and the anterior impressions are triangular. An- 

 other very important difference between the two 

 genera is the striated teeth and sockets of Parmorthis, 

 which are very rare in the whole brachiopod class. 

 It is clear, therefore, that Resserella and Parmorthis 



