MANUAL OF NATURAL HISTORY. i 63 



IT. S UB-CLASS. — Helictopod-Brachiopods 

 (Helictopoda). 



Oral arms regularly spirally- twisted when at rest ; 

 shell not pierced with minute perforations. 



I. ORDER — Sclerobrachiate-Brachiopdos 

 (Sclerobrachiata). 



Oral arms supported by a shelly plate arising 

 from the hinge-margin of ventral valve. 



1. Family. — Beaked Lamp-Shells (Lampasidse). 

 Arms spiral, supported only by short curved 

 processes ; shell not punctate, usually tetra- 

 hedral and sharply plaited. (Rhynchonellidce, 

 Auct.) 



II. ORDER. — Sarcicobrachiate-Brachiopods 

 (Sarcicobrachiata). 



Oral arms fleshy, without any shelly support ; 

 lower valve of shell simple, or with a slight median 

 elevation. 



1. Family. — Skull-Shells (Craniidse). Arms fleshy, 



spiral, attached to a process in centre of lower 

 valve ; upper valve limpet-like. 



2. Family. — Disk-Shells (Biscinidse). Arms cili- 



ated, fixed to a central process of lower 

 valve, which is perforated for the passage of 

 a peduncle, or tendon of attachment ; upper 

 valve conical, simple. 



3. Family. — Duck-billed Limpets (Lingulidse). Ani- 



mal attached by a tendinous tube, which pro- 



