135 



of being concentric, in the majority of the species inclines more to radiation, 

 and in some species the radiating ribs are profusely scaled after the manner 

 of Pholas. Though few in number, the Petricola are very generally dis- 

 tributed, one species inhabiting Britain. 



1. amygdalina, Sow. 



2. bicolor, id. . 



3. bidentata, Gray. 



4. bipartita, Desk. 



5. Chinensis, id. 



6. concinna, Sow. 



7. costellata, Lam. 



8. cultellus, Desk. 



9. dactylus, Sow. 



Species. 



10. denticulata, Sow. 



11. discors, id. 



12. divaricata, Chemn. 



13. elegans, Desk. 



14. elliptica, Sow. 



15. flabagella, Lam. 



16. gracilis, Desk. 



17. lapicida, Chemn. 



18. lithophaga, Retz. 



19. nivea, id. 



20. Pholadiformis, Lam. 



21. robusta, Sow. 



22. semilarnellata, Lam. 



23. serrata, Desk. 



24. solida, Sow. 



25. typica, Jonas. 



Figure. 



Petricola Pholadiformis. PI. 39. Pig. 212. Shell showing its atte- 

 nuated form and scaled radiations of sculpture. 



Genus 3. SAXICAVA, Be Bellevue. 



Animal ; oblong or claviform ; mantle closed in front except for 

 the passage of a digitiform foot, which is furnished with a 

 byssal groove ; siphons united nearly to the extremity, orifices 

 large, margined with a fringe of cirrhi. (Forbes.) 



Shell ; generally oblong, irregular, gaping, hinge furnished in some 

 stages of growth with two cardinal teeth, mostly toothless. 



Saxicava curiously resembles Gaslrochana in having the mantle closed 

 in front, except for the passage of a narrow finger- like foot, but its shell 

 is of a more solid Fenerupis-like growth, and the animal does not form a 

 tubular sheath. The Saxicava are borers, but not always. If there be a 

 crevice, it has been observed by Porbes and Hanley, in rock, shell, coral, 

 or seaweed, into which they can thrust themselves, they do so ; and if near 

 a limestone rock, they perforate it, and form crypts in which to live.* 



* " Mr. Osier states that when young they are very active animals, and soon commence to per- 

 forate. Both that gentleman and Mr. Garner have noticed that their excavations are not round, 

 nor the sides smoothed off, like those of the holes made hy Pholas. As for us, we only know of 

 their boring into calcareous rocks ; but Mr. Clark has noticed an instance of their perforating 



