﻿T. Davidson — What is a Brachiopod ? 205 



Tretenterata, King. 



Family 1. Lingulidce. — Shells generally either oblong or circular, 

 with a peduncle, sometimes of considerable length, passing out 

 between the valves or through a narrow channel in the hinge - 

 margin ; texture horny and calcareous : no calciiled support for the 

 labial appendages, the fleshy spiral coils directed upwards. This 

 family would comprise the following genera : Lingula, Lingulella, 

 Lingulops, Lingidepis, Glottidia, Monoholina, Obolus, Oholella, Dig- 

 nomia, Sclimidtia, Acrites, Volborthia, and some others. It may, 

 however, be hereafter desirable to group the genera above recorded 

 into two groups or families, Lingtdidce and OboUdce. Zingulella is 

 one of the oldest, if not the oldest type of animal life now known, 

 while Lingida appeared for the first time about the middle of the 

 Cambrian period, and has continued to be represented up to the 

 present time. 



Family 2. Discinidce. — Shells more or less circular or oval-shaped, 

 attached by a peduncle passing through a foramen in the venti'al 

 valve ; shell calcareous and horny, setae extremely long, barbed with 

 cilia of great length : labial appendages fleshy, curved backwards, 

 with small terminal spire directed downwards, as in Crania. Genera: 

 Discina, Tremates, Discinisca, Kutorgina (?), Acrotreta (?), Siphono- 

 treta (?). Discina appeared about the middle of the Cambrian period, 

 and has continued to exist up to the present time. 



Family 3. Craniadts. — Shells orbicular or limpet-like, entirely 

 free or attached by a greater or lesser extent of the under surface 

 of their ventral valve : labial appendages spirally coiled, directed 

 towards the bottom of the dorsal valve : shell calcareous, perforated 

 by minute canals. Genera : Crania, Craniops, Craniscus, Pliolidops. 

 The genus Crania appeared for the first time during the Silurian 

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



Family 4. TrimerellidcB. — Shells transversely or longitudinally 

 oval : ventral valve usually the largest and flattest, with a more or 

 less developed beak and area : ventral valves generally the most 

 convex, hinge rudely or faintly dentated : all the genera are provided 

 with a solid or vaulted muscular platform in the interior of both 

 valves ; no calcareous support for the labial appendages : shell cal- 

 careous, and in two of the genera very massive. All the forms are 

 extinct. Genera : Trimerella, Monomerella, Dinobolus. The genera 

 and species of this family are restricted to the Silurian period. 



Clistenterata, King. 



Family 5. Terebratididie. — Shells very variable in shape, with a 

 prominent beak, truncated by a circular foramen, jjartly completed 

 by a deltidium in one or two pieces ; labial appendages united to 

 each other by a membrane, variously folded upon themselves, and in 

 some genera spiral at their extremities. These appendages are 

 entirely or partially supported by a calcified process assuming a 

 great variety of shapes. All the species lived attached to submarine 

 bodies by the means of a peduncle. Shell structure perforated by 

 canals. Genera: Terebratula, Terebratulina, Waldheimia, Terebratella, 



