356 MOLLUSCOIDEA 



PHYLUM V 



only a portion of their existence, or cemented ventrally. Valves ventral and dorsal. 

 In composition, phosphatic or calcareous or both. Animal enveloped by two pallial 

 membranes intimately related to the shell. JFithin the mantle cavity at the sides of the 

 mouth are inserted the two, more or less long, oral, usually spirally enrolled, cirrated 

 brachia, which are variously modified, and are supported in the Terebratulacea and 

 Spiriferacea by an internal calcareous skeleton, or brachidium., attached to the dorsal 

 valve. Anus present or absent. Central nervous system consisting of an oesophageal 

 ring with weakly developed brain and infra-oesophageal ganglionic swellings. Blood- 

 vascular system probably present with the sinuses developed into vascular dilatations 

 at the back of the stomach and elsewhere. Sexes separate. Exclusively marine. 



The class begins well represented in the Lower Cambi'ian, attains its 

 maximum development in the Silurian and Devonian, and is represented by 

 about 158 living species. Probably upwards of 7000 fossil and recent species 

 have been described ; these are distributed in 450 genera, grouped in 42 

 families, 14 superfamilies, and 4 orders. 



Cuvier (1792 and 1802) was the first to distinguish the Brachiopods from 

 the Acephala, and created for them a fourth family in his class of Molluscs. 

 To Dum6ril (1806) we owe the now generally accepted class name Brachio- 

 poda, or arm-footed animals. Since the arms, or brachia, are not homologous 

 with the foot of Molluscs, Gray (1821) changed the name to Spirobranchiopoda ; 

 Blainville (1824) to Pa/to5rowc/wate ; Risso (1826) to Branchiopoda ; Broderip 

 (1839) to Brachiopodidae ; Agassiz (1847) to Branchionopoda ; Bronn (1862) to 

 Brachionocephala ; Paetel (1875) to Branchionobranchia ; and Haeckel to Spiro- 

 branchia. None of these has displaced Dum^ril's term, though the latter is 

 founded on a false physiological interpretation of the brachia. 



External Characters : Form. — The shells of Brachioj^ods are very 

 variable in form. Usually both valves are convex, but they may be nearly 

 flat, with the interior cavity extremely shallow, or the dorsal valve may be 

 concave and follow closely the curvature of the convex ventral valve. The 

 ventral valve may be cone-shaped, with the dorsal operculiform, or the 

 former may be so modified by cementation as to assume the apj:)earance of a 

 Cyathophylloid coral. The shell is commonly rostrate, with the ventral beak, 

 or apex, more or less incurved over that of the dorsal valve, or the valves 

 may be very greatly extended transversely. In fact, the form of the shell of 

 Brachiopods is so variable that, as a rule, no greater value than specific can 

 be attached to this feature. 



Fixation. — The animal is generally attached to extraneous objects by a 

 muscular pedicle which projects either from between the contracted posterior 

 margin of the two valves (Fig. 536, A), through an opening in or under the 

 beak (Fig. 535, B), or through the ventral valve (Fig. 556, A). With 

 increasing age, however, the pedicle opening frequently becomes closed, and 

 the pedicle itself atrophied. The animal may then be anchored by spines 



Schellwien, E., Die Fauna der Trogkofelschicliteu. Abliandl. d. k.k. geol. Reichs-Aiist., 1900, 

 vol. xvi., pt. 1. — Skupin, II., Die Spiriferen Deutsclilands. Geol. -pal. Abliandl., 1901, vol. viii. — 

 Tschernyschew, T. , Die obercarbouischen Bracluopoden d. Ural iind d. Timan. Mem. Comite Geol. , 

 1902, vol. xvi., No. 2. — Girty, G. H., The Guadalupiau Fauna. Profess. Paper 58, U.S. Geol. Surv., 

 1908. — Buckman, S. S., Brachiopod Nomenclature. Ann. Mag. Nat. Hist. (7), 1906, vol. xviii. 

 — The Genotype of Terebratula. Ibid., 1907, vol. xix. — Brachiopod Morphology : Cincta, Eudesia, 

 and the Development of Ribs. Quart. Journ. Geol. Soc. London, 1907, vol. Ixiii. — We.ller, S., 

 Internal Characters of some Mississippian Rhynchouelliform Shells. Bull. Geol. Soc. America, 

 1910, vol. xxi. — Genera of Mississippian Loop-Bearing Brachiopoda. Journ. (Jeol., 1911, vol. xix. 



