198 PERMIAN FOSSILS. 



the pallia! line is even less inflected than it is in J. elegans, the species just referred to. 

 The low or proximo-ventral position of the anterior adductor muscular impressions, 

 ^vhich is striking compared with the position of the same impressions in Mya, Lutraria, 

 FanopcBa, and some other genera, appears to be indicative of Allorisma having been 

 more a surface-creeping, than a decidedly burrowing genus. That the position of the 

 anterior adductor muscle is influenced by the terebrating habits of the mollusk, is 

 strongly evidenced by the fact, that in the decidedly burrowing genera, Pholas, Teredo, 

 and Xylophaga, this muscle is situated considerably above its usual situation, — so high in 

 some, that the surfaces to which it is attached, or the anterio-cardinal margins of the 

 valves, project externally in front of the umbones, and are even in some species, such as 

 Pholas dactylus and F. Candida, reflected considerably over them. Thracia puhescens has the 

 anterior adductor muscular impressions as low as in Allorisma. Perhaps the foregoing- 

 observations may induce those, possessing the opportunity, to endeavour to ascertain 

 if the habit of this species is confirmatory of the view herein taken of that of Allorisma} 

 The present genus, supposing the so-called Pholadomya Mimsteri, D'Archiac and 

 De Verneuil, to be a species, is not known to occur in earlier than the Devonian rocks : 

 it is rather common in the Carboniferous ; meagrely represented, at least as far as is 

 known, in the Permian ; and anything but abundant, taking Myacites to be the same, 

 in the Triassic deposits. I have not yet been able to satisfy myself of its existence in 

 any rocks of a later age. 



Allorisma elegans, Kiny. Plate XVI, figs. 3, 4, 5. 



Allorisma elegans, King. De Verneuil, Bull. Soc. Geol. de France, 2"^ s6rie, t. i, p. 30, 



1844. 



— — ,, Ann. and Mag. Nat. Hist., vol. xiv, p. 316, 1844. 



— — „ Geol. Russ., vol. i, p. 223, 184.'5. 



(?) Amphidesma lunulata, Eeyserling. Petschora-land, p. 258, pi. xi, fig. 16, 1846. 

 (?) Cypkicaedia bicarinata „ Op. cit., p. 257, pi. x, fig. 17. 



Allorisma elegans. King. Tennant, Strat. List, p. 88, 1847. 



— — „ Catalogue, p. 12, 1848. 



Sanguinolites elegans. King. Howse, Trans. T. N. F. C, vol. i, p. 243, 1848. 

 PanopjEA lunulata, Keyserling. Geinitz, Versteinerungen, p. 8, pi. iii, figs. 21-2, 1848. 



Diagnosis. — " Form very inequilateral : both ends closed ; anterior one the shortest, 

 and oblique superiorly ; posterior one rather square : umbones somewhat gibbous : dorsal 

 slopes with a faint angle running from the umbone to the posterior end of the shell : 

 surface slightly wrinkled transversely, and crowded with minute pimples : pallial sinus 

 somewhat shallow." (King.)^ 



1 I have seen several hundred specimens of Allorisma sulcata in situ, that is, in a bed of shale in Redesdale ; 

 but I have never seen any in an upright or inclined position, so as to indicate that they were a burrowing 

 species. 



2 Catalogue, p. 12, 



