PLATYCRINID-E. PLATYCRINITES. ' 



and Mr. Fliillips has described several species as yet more recently discovered in the 

 Devonian rocks at Newton, and Plymouth. 



IMiller's reiterated assertion as to the dorso-central plate (pehas) of this genus bemg 

 tripartite, appears to be one of several instances of his adherence to error, after he pos- 

 sessed perfect evidence as to the unsoundness of his original opinion on the subject, for 

 in his OAvn collection several perfect specimens of dorso-central plates, whole and undi- 

 vided, Avere to be found. 



This error has been so imaccountably adliered to, that Mr. Phillips has, in his 

 Palfpozoic Fossils, (Devon and W. Somerset) PI. 16, fig. 42, jtage 212, fomided a new 

 genus, Adclocrinus, on the imaginary peculiarity of an "midiAdded pelvis." Oui- cabinet 

 contains numerous specimens of undivided dorso-central plates of Platjfcrinites ; and it 

 is far from improbable that the Adelocrinus Hj/strLv wUl eventually prove to be identical 

 with Platj/crinites inter scapular is. 



As Ave differ in opinion from all preceding AVi-iters on tliis subject, some further 

 explanation is requisite, lest it might be supposed that we have arrived at a conclusion 

 unsupported by satisfactory evidence. 



Mr. Plullips, CuA-ier, Goldfiiss, and other Aviiters, have aU adopted Miller's error; 

 and had we not fortunately discovered many undiAided dorso-central plates, Ave should 

 have probably been AAdthout conA"incing eAddence of the mistake. We were first led to 

 doubt the correctness of former observers by finding several dorso-central plates, Avhich 

 had been exposed to the wash of the sea, perfectly smooth and unbroken. These plates 

 never exhibited the least indication of divisions, and a more extensiA'e inquiry confirmed 

 us in the oi)inioli that no divisions had ever existed in them. 



The circumstance that divided plates are fi-equently met AA'ith, may be explained by 

 the fact that five mternal furroAvs, or grooves, run from the central perforation in the 

 plate to its outer edge. These fiu-rows probably follow the dii-ection in which the 

 principal muscles played ; why three of them should be deeper than the remainmg tAvo, 

 is not so easily accounted for ; but it is along these three deep fiuTows, where the plate 

 is thinnest, and therefore most liable to fi-acture, that it invariably jields to pressure, or 

 falls asunder by exposure to the atmosphere. 



If a careful examination is made of the supposed divisions in the dorso-central plate, 

 (pelvis) the edges Avill be fovmd jagged and imeven, and quite dissimilar in appearance 

 to the regular divisions betAveen the other plates, clearly indicatmg that the separation 

 is the efl'cct of mechanical Aiolence, and not that of organic structure. Sometimes the 

 fictitious divisions run in a cm'ved direction, in other specimens the fissur'es rim ob- 

 liquely to the axis of the columnar point of attachment, and in no two mstances are the 

 divisions fomid to correspond Avith each other, either in form or size. 



In collecting many of these plates fi'om the strata in sihi, we have never met Avith a 

 diA'ided one. In decomposed rock it may be otherwise, but this cannot militate against 



