He observed that some fossils, "such as ammonites, gryphites, belemnites, and other shells, are either of unknown species or found only in the Indian and other distant seas" (Lyell's _Principles_).
Geikie estimates very highly Guettard's labors in palaeontology, saying that "his descriptions and excellent drawings entitle him to rank as the first great leader of the palaeontological school of France." He published many long and elaborate memoirs containing brief descriptions, but without specific names, and figured some hundreds of fossil shells.
He was the first to recognize trilobites (Illaenus) in the Silurian slates of Angers, in a memoir published in 1762.

Some of his generic names, says Geikie, "have passed into the languages of modern palaeontology," and one of the genera of chalk sponges which he described has been named after him, _Guettardia_.

In his memoir "On the accidents that have befallen fossil shells compared with those which are found to happen to shells now living in the sea" (Trans.Acad.

Roy.
Sciences, 1765, pp.

189, 329, 399) he shows that the beds of fossil shells on the land present the closest possible analogy to the flow of the present sea, so that it becomes impossible to doubt that the accidents, such as broken and worn shells, which have affected the fossil organisms, arose from precisely the same causes as those of exactly the same nature that still befall their successors on the existing ocean bottom.