30 Mr. W. King on certain Genera 



Phill., since they agree with the typical species of Atrypa in being 

 furnished with spiral appendages. 



Without being aware that so important a part as the spiral 

 characterized them, Dalman included in the genus his Atrypa 

 tumida and A, prunum*, both of which in external characters 

 approximate the two species cited at the close of the last para- 

 graph. 



M. Verneuil, in merging Atrypa into Terebratula, has been in- 

 fluenced by an opinion that few will now contend for : he sup- 

 poses that the spirals found in the shell of the former are the 

 same as the labial processes belonging to the mollusk of the lat- 

 ter f, whereas they are merely the supports of these processes, and 

 therefore homologous with the internal armature of Terebratula. 

 The figures which Mr. M'Coy has given of the spiral appendages 

 of Spirifer, &c, in the ' Synopsis of the Carboniferous Fossils of 

 Ireland/ p. 127 &c, clearly show that they are attached to the 

 hinge of the imperforate valve, which could not be the case if they 

 had been the labial processes themselves. 



The armature of Terebratula and the spirals of Atrypa having 

 been shown to serve the same office, it may be maintained that 

 this still shows the necessity of discarding the last genus. There 

 would have been some grounds for this if Atrypa possessed an 

 internal apparatus as variable as that of Terebratula, but consi- 

 dering the constancy of form of the spiral appendages, and their 

 persistency over an extensive number of shells (that is, the 

 Atrypas in the present case) related to each other by affinity and 

 geological age, it is impossible to consider them otherwise than 

 as constituting a character which separates the shells under con- 

 sideration generically from the Terebratulas. 



Atrypa is distinguished from all the spiral-bearing genera by 

 the general absence of an area % and the frequency of a foramen ; 



* I have Swedish specimens of these species exhibiting the spiral coils. 



f Geology of Russia, vol. ii. pp. 47, 48, &c. 



% As several new terms are used in this paper to express various parts of 

 a Palliobranchiate shell, and as several old ones are somewhat differently 

 employed to what they are in general, I embrace the present opportunity 

 of entering into the following explanations : — Palliobranchiate shells gene- 

 rally articulate by means of two teeth or " condyles" situated on the hinge 

 of the foraminal or " dorsal " valve, and a pair of depressions or " sockets " 

 excavated in the corresponding part of the opposite or " ventral " valve. 

 The two plates seen in the rostral or umbonal cavity of the Spirifers, &c. 

 have been described by Von Buch as " les lamelles de soutien des dents," 

 because they are connected with or appear to support the condyles : the ex- 

 pression may therefore be conveniently translated into " condyle plates." 

 On the dorsal valve of Spirifer, Lepfcena, Strophomena, Thecidea, Martinia, 

 &c, and in certain species of other genera, as Terebratula iruncata, llypo- 

 thyris rostrata, &c, are to be seen two flat spaces, one on the outer side of each 

 of the condyles ; these spaces constitute what is generally called the " area." 



