34 Mr. W. King on certain Genera 



cies, but as the former are those first described by Bruguiere, 

 Lamarck and others, I am led to think that its retention for them 

 will meet with general approval. Respecting the names Hypo- 

 thyris, Phill., and Cyclothyris, M'Coy, for the plicated species, 

 the former having the priority ought to have the preference. 



My next object will be to attempt to define the limits of the 

 genera Terebratula and Hypothyris. Besides the smoothness or 

 plication of the valves, the apical or subapical position of the 

 foramen, and the perforate or imperforate texture of the shell, 

 there are other characters which appear to be generally useful in 

 distinguishing these genera ; thus the species of Hypothyris may 

 in general be distinguished from those of Terebratula by the 

 greater or less sinuosity of their frontal margins. In the synop- 

 tical table, Terebratula psittacea is placed in Hypothyris from its 

 possessing an acute apex, a subapical foramen, and an imperfo- 

 rate shell-tissue : this species suggests the probable existence of 

 other distinguishing characters ; for example, in Hypothyris the 

 internal apparatus may be simple, and the labial appendages of 

 the mollusk attached only at their base : judging of existing Te- 

 rebratulas, the latter are attached nearly throughout their entire 

 length to a complicated apparatus. We are not yet in a position 

 to urge these differences with any degree of certainty, particu- 

 larly with respect to the labial processes, since, of the genus 

 Hypothyris, the animal of only a single species (H psittacea) is 

 known ; and as regards the apparatus, we are still but imperfectly 

 acquainted with its structure in the fossil species of either genera. 

 In Hypothyris the armature appears to be exceedingly simple, 

 consisting only of two disunited processes passing from the hinge 

 of the imperforate valve into the cavity of the shell ; but in Tere- 

 bratula these processes are united anteriorly, thereby forming a 

 loop, which is more or less folded and complicated according to 

 species*. Much care is required in ascertaining whether the dis- 

 united processes and the loop are really distinguishing characters 

 in these genera, since in fossil species the latter may be broken in 

 such a manner as simply to exhibit its two crura, which will then 

 resemble the former. M. Vemeuil has evidently been misled by 

 a circumstance of this kind in representing Terebratula eloncjata 

 with disunited processes t, as several specimens of this species in 

 my collection clearly exhibit it furnished with a folded loop ; and 

 I suspect that Mr. M'Coy has been similarly deceived in stating 



* Tn Terebratula dorsata the loop is simply attached to the crural base ; 

 in T. chilensis it is attached both to the crural base and to the anterior part 

 of the mesial plate ; in T. rosea it is only attached to the mesial plate.^ The 

 apparatus of the last species explains the erect forked process seen in the 

 centre of the ventral valve of T. natalensis. 



•}• Geology of Russia, vol. ii. 



