128 MAZATLAN BIVALVES 



intermediate in texture between the shell and the calcareous 

 incrustation. This deposit exten is about half way down the 

 burrow. This species and L. plumula are always found close 

 to the outside of the matrix ; but they do not affect the same 

 situations. In Patella Mexicana, L. aristatus is often very com- 

 mon. In about one fifth of an aged limpet Mr. Darbishire found 

 13 large specimens, without reckoning empty burrows. Here 

 however, and in Chama, L. plumula is extremely rare ; while 

 in the thick lower valves of Spondylus it is not uncommon. 

 The burrows are shaped nearly as the shell, with no power of 

 rotatory motion, and very little scope for opening the valves. 

 Traces of the foot mark are very rare. The shell, when adult, 

 is more or less incurved ventrally, and angled dorsally ; tumid 

 at both extremities. Shell (without epidermis and incrustation) 

 extremely thin. The largest specimen found perfect measures 

 long, (with the beaks) 1'56, lat. "47, alt. "4. 

 Hob. Senegal and West Indies ; found in ballast, London 

 roads (!), Forbes. Senegal, in shells of Balani, Adanson. 

 Abundant in Ostrea iridescens, W. coast Africa, StutMury : 

 do. do. Anamaboa, B. M. Guinea, Tarns. Red Sea, 

 Dunker. St. Thomas, Sornbeclc. Mazatlan ; abundant in 

 Chamse, Spondyli, Ostrea iridescens, Patella Mexicana and 

 discors, Imperator unguis and olivaceus, Strombus galea, 

 &c ; L'pool fy Havre Coll. 



Tablet 581 contains 11 pairs, and 8 pairs of valves, extremely 

 young. 582, 5 pairs and 4 pairs of valves, a stage older. 583, 

 8 pairs and 3 pairs of valves, do. 584, 6 sp. adolescent. 585, 

 5 do. adult. 



Tablet 586 contains 6 young sp. in situ, burrowing in Patella 

 Mexicana. 587, 1 do. do. with the beaks detached, shewing the 

 glossy epidermis beneath. 588, 2 adult sp. in P. Mexicana. 

 589, fragment of P. Mexicana, displaying shelly lining to 

 tubes. 590, another fragment, in which a Lithophagus had 

 bored '6 beyond the inner surface ; the limpet having defended 

 itself from its pursuer by fresh layers of shell. 591, 4 sp. of 

 Patella discors, variously distorted by Lithophagi. 592, Fis- 

 surella rugosa and F. alba, similarly attached. 593, Mytilus 

 tenuiaratus, with 3 Lith. in situ : part of the mussel being 

 broken away displays its jasper-like texture, as well as the 

 shelly lining of the tube. 594, 3 sp. of Imperator unguis, with 

 Lith. in situ ; in one, they have just reached the interior of 

 the mouth ; another is bored across the apex ; the other, not 

 finding room within, has increased its size by raising a tumulus 



