MYTILUS. 113 



dermis of immature shells is yellowish-brown, and has 

 an agglutinating property, being frequently coated with 

 gravel and bits of shell. In this state the dorsal angle 

 is more acute. Rude nests or cases are occasionally 

 made by the young for their protection. The spots 

 where pearls are in course of formation exactly corre- 

 spond with the holes drilled by species of the curious 

 sponge called Cliona from the outside of the shell. A 

 transverse section admirably displays the mode of growth 

 and secretion of colour. The outside layers are purple ; 

 while the inner layers, which are three times as thick 

 and numerous as the others in full-grown shells, are 

 white. The " horse-mussel," as it is called, attains an 

 enormous size under favourable circumstances. One of 

 my specimens, which Professor King got on the Nor- 

 thumberland coast, is about nine inches and a quarter 

 long and of a proportionate breadth and capacity. It 

 would have made a dainty drinking-cup for Mimer, or a 

 pretty toy for one of the other giants, of whom Ohlen- 

 slager sung in his ' Nordische Guder/ 



" And all round the cavern might plainly be seen, 



Where Giants had once been at play ; 



For the ground was with heaps of huge muscle-shells strewn, 

 And strange fish were mark'd in the clay." 



This is the Modiola papuana of Lamarck, but not the 

 Mytilus Papuanus of D'Argenville, which is a tropical 

 species. The former referred to his Modiola tulipa the 

 Mytilus modiolus of Linne. Mr. Hanley also states that, 

 from an examination of the typical specimens, the M. 

 modiolus of Linne is not our species but the Modiola 

 tulipa of Lamarck. This shows the discrepancy between 

 the Linnean collection of shells and the ' Systema Na- 

 turae/ and how little reliance can now be placed on the 

 former for identifying some of the species. Linne's de- 



