276 LECTURES ON MOLLUSCA. 



were probably attached by the long hollow spines, which adorn the 

 shells ; and may have moored themselves in chinks, or partly buried 

 in mud. Productus has the hinge-line linear, and is a Devonian 

 group. Aulosteges has a hinge-area, like Spondylus. Strophalosia 

 was attached by the beak of the front valve. The Silurian Chonetes 

 has one row of spines along the hinge-line of the front valve. 



Family Craniad^e. 



The Cranias have lived from the palasozoic times till now. They 

 have no hinge, and are attached by the front valve : the back valve 

 being limpet shaped. The mouth-arms are free, supported by a nose- 

 like projection in the front valve. The eye-like muscular scars give 

 some of the species a rude resemblance to a skull. The valves are 

 shelly, and very minutely punctured. The ancient Pseudocrania had 

 the valves free. The position of Spondilobolus is uncertain. 



Family Discinidje. 



The shells of Piscina are quite horny, and flexible when fresh. 

 They are attached by a peduncle, passing through a chink in the 

 lower valve. The mantle is surrounded by stiff bristles ; but the cilia 

 on the mouth-arms are very tender and flexible. The ancient fossils 

 have been separated as Orbiculoidea. Trematis has convex valves, with 

 a thickened hinge-margin. • Siphonotreta is covered with hollow spines, 

 with a tubular hole at the beak. Acrotreta is shaped like Calceola. 



Family Lingulid^. 



As the Lingulas are the earliest, so they may be regarded as the 

 lowest bivalve shells. They live half buried in sand or mud, often at 

 slight depths ; and, as their horny shells hang at the end of a very 

 long peduncle, they have no slight resemblance to the Lepad Barna- 

 cles. Members of the group lived in all ages in the British seas, down 

 to the Coralline Crag ; and a species is still living on the Atlantic 

 shores of North America. The Silurian form Obolus is nearly round, 

 with a thickened hinge-margin. 



CLASS TUNICATA. 



{Tunicaries, or Cloaked MollusJcs.) 



We have now completed our sketch of the shell-bearing classes of 

 Mollusks. The remaining groups form a transition to the zopphytic 

 condition of animal life. The higher Tunicaries offer many points of 

 similarity with the sedentary Lamellibranchs ; but the lower races 

 lose their separate individuality, and become incorporated into a gen- 

 eral mass of life, like the Polypes. Although not attractive to the 

 general observer, they present many points of singular interest to the 

 scientific student. They have lately been carefully examined and re- 

 ported on by Huxley and Rupert Jones. The first group are the soli- 

 tary or simple Ascidians. 



