18525). | Ne 
certainly seemed to accord very closely in size and shape with the upper jaw. 
Any one who has observed the very close correspondence between the dentition 
of suiline pachyderms, and the great variation in different portions of the dental 
series, will appreciate the difficulty of apportioning properly small fragments of 
closely allied species, and will excuse the faults which I have unconsciously 
committed. As the best reparation for these errors, I subjoin the following 
synonyms of the fragments of the head, described by me: 
Hyops depressifrons Lec. Am. Journal of Sc. 2d series, 5, 103. 
Dicotyles depressifrons Lec. Proc. Ac. Nat. Sci. 6, 3. 
(in part.) Platygonus compressus Lec. Am. Journal, 1. c. and’ Trans. Am. 
J\Oo ERIS, No Sik By Js) tly By Bo 
Protocherus prismaticus Lec. Am. Journal, 1. c. 5, 103: Proc. Ac. 6, 5. 
The fragments to be here referred, are a cranium: fragments of anterior part 
of head with premolars and canines : the entire dental series of upper jaw : except 
the last molar, the canine and two inferior molars of lower jaw, the latter be- 
longing to a smaller specimen of perhaps another species. 
Platygonus compressus Lec. Am. Journ. |]. c. and Trans. Am. Ac. 
Arts. 
Lower jaw with two posterior molars: pl. 3, fig. 7. | 
Upper teeth,* posterior molar and two premolars: pl. 3, fig. 12 and 13, (left 
hand.) 
e 
Mr. Lea stated some of his views regarding species, and mentioned that the 
subject had engaged the attention of eminent naturalists, some of whom had given 
definitions, which did not fulfil ‘his idea of what constituted thatterm. Lindley, 
in his Introduction to Botany, jhas given some very judicious and philosophic 
views on classification... ‘*Species,’? he says, ‘are created by Nature herself, 
and remain always the same in whatever manner they may be combined; they 
form the basis of all classification, and are the only part of it which can be con- 
sidered absolute.’’? In this he makes noattempt to define his ideas of species. It 
is assumed to be a distinct creation, and unchangeable. 
MM. Ray and Drouet (“* Revwe et Mag. de Zoologie, 1849”) give their views 
of what forms a species, in the following terms: ‘‘ Generalement on entend, par 
ce mot, (espéece) un type d’organization de forme et d’activité, rigoureusement 
déterminé, qui se’ perpétue successivement par génération directe et d’une ma- 
niére indéfinie avec la meme constance de caractéres.”’ 
Milne Edwards’ definition of species, Mr. Lea thought was less clear. He says, 
«¢ On donne le nom d’espéce Ala reunion des individus, que se reproduisent entre 
eux avec les memes propriétes essentielles.” 
Cuvier considered that the fact of the suceession, and of the constant succes- 
sion, constituted alone the validity of the species. 
Dr. Morton comprised his view of species, as “¢a primordial organie form.” 
Neither of these definitions fulfil Mr. Lea’sidea of what forms a species. It 
seemed to him, that.a species must be considered to bea primary established law, 
stamped with a persistent form pertaining solely to itself, with the power of 
successively reproducing the same form and none other. 
Mr. Lea also stated he was about to issue a new edition of his Syxopsis of the 
Family Naiades, with much additional matter.. That in the introductory part, 
he had given the classification of various zoologists, and among others those who 
had divided the Natades by their anatomical differences, viz: D’Orbigny, Tro- 
schel, and Agassiz. In these he said it would be observed, by consulting the 
works of the two first, and a work entitled ‘*Shells of New England,”? by Mr. 
*The figures of these teeth were so badly drawn by the artist engaged by me, and 
still farther deteriorated and altered while in the hands of Mr. Endicott, that the minute 
differences which distinguish such closely allied animals are not to be seen: in excuse 
for permitting such figures to go before the world, I must say that the memoir was 
printed during my absence on a scientifie expedition, and that no opportunity was offered 
for correcting the proof. 
