VENUS.— MACTRA. 



85 



8. The Venus '^^^.^.tl'''^'"''^ 



{ Ji -I A-y\ • • /LIGAMENT-. \ 



{fig' 107) IS re- 

 cognised by the 

 hinge, the ante- ^'\ 

 rior and poste- 

 rior plates of ^ ' 

 which, instead 

 of being- sepa- 

 rated from the 

 middle tooth, as -^ 

 in the Cardia, 

 &c., are approxi- 

 mated in a sin- 

 gle group be- 

 neath the sum- 

 mit. The shell Fig. 107.— venus. 

 is generally thick, moderately convex, and a little elongated 

 The species of this genus are very numerous. 



9. The genera Petricola and Venerupis (j%. 106) resemble 

 the Venus very much, and have obtained their names from living 

 in the interior of stones which they perforate. 



10. The Mactr^ differ from other cardiaceas in having an 

 internal ligament, lodged in two pits in the hinge, corresponding 



v/ith each other i^jig. 108). There 

 are some large species on our 

 own coast. 



11. The fifth and last family of 

 testaceous acephalse or Inclusa, 

 is composed of mollusks in which 

 the mantle is only open at its an- 

 terior extremity, or near the mid- 

 dle, for the passage of the foot, 

 and is prolonged posteriorly in a 

 double tube ; in other parts it is 

 completely closed {fig- 109). 

 The shell is always gaping at the 



Explanation of Fig. 107. — Cytherea, or Venus ; — s. the summit or beak ; — 

 I. the lunula ; — p. pit ; — A. anterior, and P. posterior extremity of the shell. 



8. What are the characters of the genus Venus ? 



9. From what circumstance do the genera Petrico'la and Venerupis ob- 

 tain their names? (Petricola, from the Latin, petra, a stone, and colo^ I 

 inhabit. — Venerupis, Latin, Venup,and rupes, a rock.) 



10. How is the genus Mac'tra characterized ? (Mactra, Latin, a 

 trough.) 



11. What are the general characters of the family of Inclusa? (Inclusa^ 

 Latin, enclosed.) 



Fig. 108. MACTRA. 



8 



