figured in Phillips's l Geology of Yorkshire. 1 291 



Patella retrorsa, Phillips (p. 223, t. 14. fig. 5) . 



The type of this species is retained in limestone, and is in 

 an unsatisfactory state of preservation. The undulating border, 

 corresponding concentric strise, segmented sides, and depressed 

 form appear to be characteristic points of the species, although 

 the grooves running up the flanks of the shell are rendered 

 too definite in the figure. It is the 



Patella retrorsa, Phillips, loc. cit.j Morris, Cat. Brit. Foss. 1854, 2nded. 

 p. 266. 



Patella lateralis, Phillips (p. 223, t. 14. fig. 6) . 



Although this is stated to be in the Gilbertson collection, I 

 have not met with the specimen. 



On the Shells called Patella by Phillips. — No conclusive 

 evidence, so far as I am aware, has been adduced to show in 

 what relation these old Patelloid shells stand to the genus 

 Patella as now understood. Of the muscular impressions we 

 know little or nothing ; and it appears to me, in consequence, 

 that it would be better to refer them to some other genus, 

 pending further details of their structure, rather than to defi- 

 nitely place them in a genus now existing, and to which they 

 may perhaps bear no other affinity than that of outward 

 resemblance. 



Prof. James Hall has proposed* for American Lower- 

 Silurian shells of somewhat similar aspect the name Palce- 

 acmosa, but which he defines as possessing an entire and non- 

 sinuate peristome. With the exception of this one character 

 his definition would quite accord with the shells referred by 

 Phillips to Patella ; and it becomes a question whether it 

 would not be better to enlarge Hall's diagnosis, so as to 

 include shells with both a plain and sinuated border or peri- 

 stome. In this case the above species would become Palce- 

 acmma sinuosa, Phill. sp., Palo3acma3a curvata, Phill. sp., 

 and so on. 



Hall's definition is as follows : — " Conical univalve shells, 

 having a circular, ovate, or elliptical outline, with a more or 

 less elevated subcentral apex, either erect or slightly curving 

 towards one extremity ; peristome entire, not sinuate. Sur- 

 face marked by concentric ridges of growth. Internal mus- 

 cular markings unknown." 



On the other hand, if this alteration of Hall's diagnosis is 

 not permissible, these shells must remain as simple Patellar 



* Twenty-third Ann. Report New-York State Cab. Nat. Hist. 1873, 

 p. 242. 



