]48 SALENIDiE. 



MM. Agassiz and Desor have separated from the true Salenia the genera Peltastes and 

 Hyposalenia. By the same authors a doubt has been suggested about the vahie of this 

 character, as it may be only an accidental and variable condition ; to this objection 

 M. Cotteau replies, " that he has examined more than four hundred specimens of Salenice 

 appertaining to the different stages of the Cretaceous formations, and representing almost 

 all the known species, many among which, such as Peltastes acaiithoides, P. Studeri, Salenia 

 petalifera, 8. Prestensis, and S. Bourgeoisi present characters that are clearly defined. 

 Nevertheless we have recognised in each of those species that the periprocte, whether 

 in the axis of the animal or out of the axis, occupies a place which is invariably the same." 



The structure of the apical disc, and the lines and impressions marked thereon, afford 

 a character of secondary importance, for although the general outline of these sutures is 

 remarkably constant, still there are exceptions which show that caution must be exercised 

 in using it, along with others, in the determination of species. The size of the ambulacra, 

 and the number and magnitude of the granules contained therein ; the form of the 

 test, the size of the inter-ambulacral tubercles ; the dimensions of the oral aperture, the 

 structure of the peristome, and the depth of the incisions or entailles, collectively form 

 good specific characters on which we can rely. 



M. Cotteau divides the family Salenid^ into six genera, of which he gives the 

 opposable characters in the following table.^ 



A. Ambulacra large, straight, furnished with small tubercles; apical disc^ 



with a compound sur-anal plate, the elements of which are covered C Acrosalenia. 

 with granules. \ 



B. Ambulacra narrow, sub-undulated, furnished with granules; apical 



disc large, prominent ; sur-anal plate single ; elements large shield- 

 shaped, marked by regular impressions. 



a. Tubercles perforated. 



X. Periprocte excentral and posterior, situated in the 



1 



XX. Periprocte excentral and posterior, situated with- ) 



„ ^, . , , PSEUDOSALENIA. 



axis 01 the animal 



, ^, . „ , , , ( Heterosalenia. 



out the axis or the body ) 



b. Tubercles imperforate. 



X. Periprocte excentral and posterior, situated in the") 

 axis of the animal. I 



1. Ambulacra destitute of poriferous impres- f 



sions J 



2. Ambulacra furnished with poriferous im- ") 



> GONIOPHORUS. 



pressions J 



XX. Periprocte excentral and posterior, situated with-^ 



out the axis of the body and incUned to the (. Salenia. 

 right side \ 



1 « 



Paleontologie Fran9ai8e ; Terrain Cretac6,' torn, vii, p. 90. 



