The Austral Rhynchonellacea. 179 



forms of beaks in Mesozoic rhynchonellids, that there is a direct 

 relationship in this part of the structure with the Tertiary forms. 

 The foramen also in " R. variaMlis is distinctly hypothyrid and sub- 

 elliptical, and the deltidial plates, though small, are very similar in 

 shape to those of Tegulorhynchia. " R." variabilis has the conjunct 

 type of deltidial plates, whereas in the form of Tegulorhynchia they 

 are almost invariably disjunct (exception, T. Squamosa) . 



(d). — Variation in Time. 



The majority of the Jurassic and Cretaceous forms in Australia 

 (R. rustica, eyrei^ croydonensis-pars, solitaria and variaMlis) agree in 

 their main characters with Buckman's Burmirhynchia. His definition, 

 taking Burmirhynchia gutta, nov. as type isio — " hypothyrid (beak 

 massive, springing from a gibbous umbo, apex produced and incurving 

 [foramen sub-elliptical, deltidial plates narrow, disjunct]); slightly 

 trilobed; multiplicate; dental plates strong, much divergent; ventral 

 muscle area large, pyriform; dorsal septum strong; dorsal muscle area 

 quadriform to subcircular pattern, the two anterior scars strongly 

 marked, making a cordate figure, being individually more or less 

 pyriform, bounded by well-marked, diverging channels." 



In these early forms the dorsal septum is strong. The later, 

 Tertiary and living representatives, have a low dorsal septum, but 

 extended, probably pointing to an ancestral feature of greater de- 

 velopment. The dorsal muscle areas agree in shape both in the Indian 

 and Australian Jurassic, and also in the Australian Tertiary types, 

 though in the latter they are of greater extent. 



In some of the earlier Mesozoic forms the beak has already been 

 modified by truncation; that is to say, less incurved and prominent. 

 The deltidial plates are disjunct, both in Burmirhynchia and KaU 

 lirhynchia, and these show features ancestral to the austral form, 

 but common to both. The tegulate character is seen developing In the 

 early forms to some extent, but not until the Miocene is it well 

 pronounced. 



(e). — Palaeographical Factors. 



The Burmirhynchia type of shell was already established in the 

 Lias, Inferior Oolite and Greater Oolite of England, and the Con- 

 tinent. It appears to have migrated along a possible shore-line in 

 Upper Jurassic times, judging from the community of Upper Jurassic 

 types of mollusca and brachiopoda seen in the Western Australian 

 beds, where foraminiferal species are also common to both areas. 



During this time also, the terrestrial areas were continued, form- 

 ing a favourable geographical unit from Europe across to India, and 

 thence to Australia. Many Jurassic invertebrates of the Indian 

 area are common to Australia. The interesting fact is here brought 

 out, that of the later type of rhynchonellid, the genus Cyclothyris 

 appears to be absent thus far, and our Australian Cretaceous species 



10. Buckman, 1917, p. 49. 



