CONCHIFERA. 425 



in crevices of rocks, and the empty burrows of boring mollnsks ; 

 tbey are often much "worn and distorted. The genus Palaarca 

 probably belongs here ; we have not been able to ascertain the 

 generic characters ; but they may be found in the Memoirs of 

 the Geol. Surv., Canada, Yol. III., under the head Cyrtodonta. 



Distribution, 140 species. World wide, most abundant in warm 

 seas; low water — 230 fathoms {A. imbricata, Poll). Prince- 

 Eegent Inlet {A. glacialis). A scaphula, Benson, is found in 

 the Ganges and its branches, from Calcutta to Humeerpoor on 

 the Jumna, 1,000 miles from the sea. A second species has 

 been found in the river Tenasserim, Birmah. The hinge is 

 edentulous in the centre, and the posterior teeth are laminar 

 and branched ; the elements of the posterior muscular impres- 

 sion are distinct. 



Fossil, 400 species. Lower Silurian — . United States, 

 Europe, South India. 



CucuLL^A, Lamarck. 



Etymology, cucullus, a cowl. 



Type, C. concamerata, PI. XYII., Pig. 14. 



Shell sub-quadrate, ventricose ; valves close, striated ; hinge- 

 teeth few and oblique, parallel with the hinge-line at each end ; 

 posterior muscular impression bounded by an elevated ridge. 



Distribution, 2 species. Mauritius, Nicobar, China. 



Fossil, 240 species. Lower Silurian. North America, Pata- 

 gonia, Europe. 



Sub-genus, Macrodon, Lycett. M. Hirsonensis, PI. XYIL, 

 Fig. 15. Shell with a few oblique anterior teeth and one or 

 more long laminar posterior teeth. The Ark-shells of the 

 Palaeozoic and secondary strata have their anterior teeth more 

 or less oblique, like Area, the posterior teeth parallel with the 

 hinge-line like Cucullcea ; their valves are close or gaping below ; 

 their umbones frequently sub-spiral ; and the hinge-area is 

 often veiy narrow, and in some species only the posterior moiety 

 is visible. 



Parallelopij)edum, Klein. The outermost hinge-teeth short, 

 and perpendicular to the hinge-line; teeth developed along 

 the whole length of the hinge. 



Pectunctjlus, Lam. 



Type, P. pectiniformis, PI. XYIL, Fig. 16. (Area pectun- 

 culus, L.) 



S'AeZZ orbicular, nearly equilateral, smooth or radiatcly striated; 

 mnbones central, divided by a striated ligamental area ; hinge 



