﻿206 T. Davidson — What is a Bmchiopod ? 



Megerlia, Kraiissina, Terehrirostra, Ifagas, Mannia, Bouchardia, 

 Platidia, Argiope, Cistella, Zellania, Heusselaria, Gwijnia, Macandrevia 

 Diclasma, Meg anther is, Stringocephalus, Trepidolopsis and some others. 

 Terebrahda appeared at the close of the Silurian period, and con- 

 tim;ed to be represented up to the present time; but the larger 

 number of genera have had a very limited distribution in time. 



Family 6, Thecidcedie. — Shells small, thick, varied in shape, 

 attached by a larger or smaller portion of the shell substance of their 

 ■ventral valve ; area flat ; deltidium indistinct ; valves articulated ; 

 loop in the dorsal valve folded into tv^o or more lobes lying in 

 hollows of corresponding shape excavated in the substance of the 

 valve. This loop, or apophysary ridge, supports the brachial mem- 

 brane, whose thickened and cirrated margin is apparently attached 

 to the inner sides of the grooves ; shell structure perforated by 

 canals. Only one genus at present known, Thecidium. It appeared 

 in the Trias, and has continued to be represented up to the present time. 



Family 7. Spir if eridce. ~8he\ls variable in shape, ovate, elongated, 

 transverse, trilobate, etc., with the hinge-line straight and extended 

 into wing-shaped expansions : valves articulated, with or without a 

 flattened area in the ventral valve : animal free, or attached during; 

 at least a portion of its existence by means of a peduncle, or by 

 muscular fibres issuing from an angular or circular foramen in the 

 beak or area of the ventral valve. Dorsal valve internally furnished 

 with two calcareous spiral processes connected in different manners 

 and directed outwards towards the side of the shell ; these processes 

 afforded support to the brachial appendages. This family comprises 

 the following impunctate and punctate genera : Spirifera, Cyrtia, 

 Cyrtina, Spiriferina, Martinia, Athyris, Meristina, Jferista, Betzia, 

 Niicleospira, Trematospira, Bhynchospira, Meristella, Zygospira, Coclo- 

 spira, Bhyncliotrema, Uncites, Amboccelia, Charionella, Syringothyris, 

 Eumetria, Suessia, Vetulina (?). The first species belonging to this 

 family made its appearance during the Silurian period, and the 

 family became entirely extinct in the Inferior Oolite. 



Family 8. Bhynchonellidcs. — Yalves articulated, very variable in 

 shape, more or less trigonal, often trilobed or ovate, smooth or 

 plicated ; foramen beneath a usually produced and pointed beak, 

 completed by a deltidium at times concealed : brachial appendages 

 fleshy and spirally coiled, flexible, and sujDported only at their origin 

 by a pair of short-curved shelly processes, or throughout by two 

 broad spirally-coiled lamellse (these spires are vertical, closely 

 adpressed, and directed upwards towards the centre of the valve): 

 shell structure fibrous and impunctate. This family comprises the 

 following genera : Bhynchonella, Atrypa, Eatonia, Leptoccelia, Bra- 

 chymerus, Anastropliia, LeiorTiynclius, Camarophoria, Ehynchopora, 

 Bliynclionellina, and one or two others. The first species appeared 

 during the Silurian period, and representatives of the family have 

 continued to the present time. 



Family 9. Pentameridce. — Shells ovate, somewhat pentagonal ; 

 valves articulated, without hinge-area ; foramen angular ; no delti- 

 dium : inside of ventral valve two contiguous vertical septa of 



