186 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



of the dorsal one being shallow and supported by a very 

 low median septum. This figure is utterly incorrect 

 and gives an entirely wrong impression of the interior 

 of Virg'iana. The dorsal structure of this genus is 

 precisely like that of Pentamerus in composition and 

 most nearly resembles that of Platymerella in the 

 brevity of its septa. 



discrete, with more or less long brachial supports. 

 The only genus is Strlcklandia Billings of the Silurian. 

 The origin of this family is not yet known, but it 

 may have come out of camerellids. Instead of retain- 

 ing the shell form of the camerelHds, however, the 

 Stricklandida; developed into wide-hinged types. It is 

 possible also that the stricklandids came out of a much 



O (D O O C) 



s 



o 



6 



10 



II 



Fig. 35. — Serial sections of Virgiana barrandei (Billings), from Anticosti. Notice the general similarity of the structure 

 to that of Pentamerus s. s., pi. 29, fig. 11. The difference between the two genera internally is the brevity of the dorsal 

 septa in Virgiana, in this specimen a little more than 2 mm. Distance from beak: 



1 — 1.0 mm. 7 — 6.4 mm. 



2—2.2 8— 7.7 



3 — 2.9 9 — 8.9 



4 — 4.4 10—10.5 



5—5.2 11—11.5 

 6—5.9 



The shell sectioned was 41.8 mm. in length. All trace of the spondylium disappeared at 24 mm. from the beak; the 

 ventral septum disappeared at about 5 mm. These facts indicate an exceedingly long spondylium and very abbreviated 

 septum. 



? Family STRICKLANDIDA Hall and 

 Clarke 1894 



Divergent large Pentameracea, probably developed 

 out of the Camerellidas. Spondylium short, supporting 

 a short septum. Cardinalia simple but specialized and 



older stock, the Syntrophiids. In neither supposition are 

 the annectant genera known, but it is more natural 

 to look for them in the later Ordovician, in which 

 event the origin would be out of the camerellids. 



The Stricklandias are first met with in the early 

 Silurian of North America in faunas of northwestern 



