176 F. Chapman and Irene Crespin : 



responding to growth-lines which cross the ribs that are typically 

 constant on valves. By structural specialisation the tiegulation be- 

 comes tubular, and eventually links up with that seen in Tegulorhyn- 

 chia doederleini. Beak hypothyrid; as seen in edge view erect, usually 

 truncate or only slightly incurved. Deltidial plates strong and tpyically 

 equilaterally triangular. Dental plates well defined and strong, the 

 septum represented on inside of dorsal valve, by a very thin plate 

 which extends to nearly one half of the length of the shell. Muscle 

 areas on interior of the dorsal valve are well marked and sub-quadrate, 

 and fairly large. 



(b). — Distinctions from Hemithyris. 



According to Professor Schuchert, Hemithyris is defined as 

 " smooth or faintly plicate rhynchonellae with a high ventral beak 

 and open delthyrium. No dental plates." As a matter of fact, dental 

 plates are present in the genotype Hemithyris psittacea, Chemnitz sp., 

 the typical northern form, as indicated by Davidson, Dall and Thom- 

 son (see pi. I., fig. 13; pi. II., figs. 16, 19). 



So far as we have seen, these dental plates are always slender 

 and not well developed. On the contrary, the forms of the southern 

 R. nigricans series are stout and strong compared with the northern 

 Hemithyris. The type of Hemithyris, d'Orbigny {Rhynchonella psit- 

 tacea) has8 a " beak sharply pointed and incurved." R. squamosa has 

 it erect and more often quite blunt. 



In regard to the ornament, Hemithyris is typically smooth, or 

 .striate, never distinctly costate, so far as we have seen from the 

 specimens in the National Museum collection, which were dredged 

 from the North Sea. In these example, also, the growth-lines are 

 purely concentric and never tend to become strongly undulose or over- 

 lapping to the eixtent of even a form like the living nigricans. In 

 Hemithyris the septum is rudimentary, low, and barely extending half- 

 way across the shell. In Tegulorhynchia the se|>tum is more strongly 

 developed, and extends into the anterior half of the shell. The 

 deltidial plates in a tj^pical Hemithyris psittacea are elongated in the 

 form of a scalene triangle, whereas in Tegulorhynchia they are almost, 

 to quite, equilateral. These plates are strongly discrete in H. psittacea, 

 whereas in Tegulorhynchia they may be only slightly so (T. coelata) , 

 or conjunct (T. squamosa). 



(c). — Phylogeny of the Genus. 



From the widely spread occurrence of the Tertiary rhynchonellids 

 of the squamosa type in the southern hemisphere, it is only natural 

 to conclude that these forms sprang from some already well established 

 ancestor in the Mesozoic. 



Of the recorded Mesozoic species found in Australia we have: — 

 Rhynchonella croydonensis, Etheridge, fil., 892, p. 560, pi. XLI., figs. 

 13, 14. 



8. Davidson, 1887. p. 164. 



