FAMILY 4. PECTIN ACE A. 163 



cardinali externa separatis. Area plana, trigona, state productior, 

 sulco ligamentifero interdum obtecto, partita. Cardo dentibus duo- 

 bus solidis, reflexis, in utraque valva, valide intersertis, ligamen- 

 tum in cavitatem inter se recipientibus. Impressio muscularis sub- 

 lateralis. 



The Spondylus or Thorny Oyster appears to have been well known to 

 the ancients ; both Aristotle and Galen having been struck by the beauty 

 of its shell, which they describe under the title of <jtt6v$v\oc or a^ovSvXoc. 

 The Spondyli are remarkably characteristic ; indeed, the present genus 

 remains nearly as entire as in the time of Linnaeus : they differ materially 

 from the Pectines, and might perhaps with more propriety be associated 

 with the Plicatulce in a separate family. The shell of Spondylus is thicker 

 and of more irregular growth than that of Pecten ; it is always attached 

 by one or both valves, and there is a marked change in the hinge, both 

 in being provided with teeth and in forming a solid umbonal area. From 

 the ponderous structure of some of them the animal appears to have a 

 most abundant supply of calcareous matter ; the peculiarity of growth 

 noted in our observations on Vulsella is here very conspicuous, for as the 

 shell advances in growth we find the valves to be composed of several di- 

 stinct plates, deposited in progressive order one upon the other. In one 

 species, the Spondylus varius, this laminar structure is very remarkable ; 

 the plates are so irregularly secreted as to admit the water between them, 

 so that when the last plate is deposited it is completely sealed up. In 

 many individuals of this species the water may be seen through the trans- 

 parency of the shell : hence they are commonly called the Water Spon- 

 dyli. 



The shell of Spondylus may be described as being attached, inequivalve, 

 somewhat irregular, and more or less auriculated. The outer surface is 

 rarely smooth ; in fact, nothing can exceed in beauty the varieties of 

 external development whether of spines or foliations ; their colours also 

 are exceedingly bright and various. The umbones are unequal, and 

 separated from each other by a flat triangular disc on the lower valve. 

 The disc is always perfectly smooth ; it increases with the growth of the 



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