108 Miss Agnes Crane — Evolution of the Brachiopoda. 



branches, one of the extinct Spiriferoid, Meristoid, and existing 

 Rhynchonelh)id, and the other of the Terebratnh)id type, still more 

 familiar. By far the largest number of Brachiopoda — half of all 

 the known genera — belong to this order of Telotremata,^ which is 

 characterized by the deltidium in two plates, and tlie presence of 

 variously modified calcified supports to the brachial or breathing 

 organs of the "arm "-footed mollusc-like animals. 



Thus, to sum up the ordinal relationships, the atrematous order 

 represents the short main trunk of the Brachiopodal phylum — the 

 Neotremata and Protremata chief divergent, but now dwindling 

 branches, while the secondary, but more robust branch of the 

 Telotremata, which originated in the Protremata, has given rise 

 to more numerous stems, branchlets, and offshoots than all the rest 

 put together. By far the lai-ger number of surviving genera and 

 species belong to this flourishing branch of the Brachiopodal family 

 tree. 



So much for the ordinal evolution of the Brachiopoda, which 

 has been indicated in the diagram already published (Plate IV.), 

 in which the top line represents the seas of the pi'esent day, and the 

 bottom one the Primordial oceans of long past ages (Geol. Mag. 

 p. 65, February, 1895). 



Let us now consider some instances of the evolution of genera. 



The simplest form of all known Brachiopoda^ is that small semi- 

 elliptical, horny, imperforate hingeless shell Paterina, the little 

 father of all the Brachiopoda. The genus was founded on a Lower 

 Cambrian species first referred to Kutorgina and Obolus by Billings 

 and Walcott,^ but assuredly not belonging to the fii'st-named genus 

 if Kutorgina cingulata, Billings, from the Middle Cambrian be taken 

 as its type, for that is a calcareous shell with indications of a hinge- 

 area, and there seems reason to consider it as an ancestral form of 

 the articulated Protremata (PL V. Fig. 10.) 



In external shape the atrematous Paterina (PI. V. Figs, la, b, c) 

 bears a close resemblance to the horny imperforate protegulum, or 

 earliest developed shell covering, which makes its appearance in the 

 phylembryonic stage (when class characters can be first determined 

 with certainty) of all larval Brachiopoda. According to Beecher, 

 nearl}' every Brachiopod goes through a "paterine" stage of develop- 

 ment (5). The Primordial little "coin-shaped" shells known as 

 Obolella (Fig. 2) pass through their transient paterine stage, and 

 the resultant Obolelloids play ar\ important part in the genealogy 

 of Lingula (Figs. 5, 7), one of the few "tongue-shaped" shells of 

 the atrematous order still existing. For the Linguloid type was 

 evidently derived from the preceding Obolelloid which had attained 



1 The earliest known forms of the telotrematous sub-orders Rostracea, 

 Helicopegmata, and Ancylobrachia approximate. There is reason to believe 

 that the Eostracea and Ancylobrachia are older than the Helicopegmata or 

 Spiriferacea. 



^ "And it might be fair to say of all possible Brachiopoda" — add Hall and Clarke 

 {op. cit. p. 912, 1895). 



^ The earliest form and type species is Paterina Swantonensis, "Walcott, sp., from 

 the Lower Cambrian of Vermont, first described as Obolus [Kutorgina) Zabradorica. 



