VENUS. MACTRA. 



85 



EXCAVATION 



OFTHEPALLIAL 



IMPRESSION 



Fig. 107. VENUS. 



8. The VENUS 

 (fig. 107) is re- 

 cognised by the 



hinge, the ante- /."I"." 

 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 

 is generally thick, moderately convex, and a little elongated 

 The species of this genus are very numerous. 



9. The genera Petricola and Venerupis (fig- 106) resemble 

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

 in the interior of stones which they perforate. 



10. The MACTR^E differ from other cardiacese in having an 

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



with each other (fig. 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 



Fig. 108. MACTRA. 



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

 I. the lunule ; 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 coZo, I 

 inhabit. Venerupis, Latin, Venus', 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.) 



