Brongniart (1822, p. 40) devoted five pages of his monograph to a discussion of the affinities of trilobites, concluding that it was very probable that the animals lacked antennæ and feet, unless it might be that they had short soft feet which would allow them to creep about and fix themselves to other bodies.
Schlotheim (1823) thought that the spines on Agnostus pisiformis were segmented and compared them with the antennæ of Acarus.
Stokes (1823) was the first who, with understanding, published an illustration of the ventral side of a trilobite, having figured the hypostoma of an Isotelus. He was followed in the next year (1824) by Dekay, who also figured the hypostoma of an Isotelus, and added some observations on the structure of trilobites. The researches of Barrande, Novak, Broegger, Lindstroem, and others have dealt so fully with the hypostoma that further references to that organ need not be included here.
Dalman (1826, 1828) reviewed the opinions of his predecessors, and thought it not impossible that organs of mastication may have been present under the head shield of the trilobite as in Limulus (1828, p. 18). In this he of course followed Wahlenberg.
Goldfuss (1828) figured sections of Dalmanites hausmanni, Phacops macrophthalma, and Calymene tristani, which remind one of some of Doctor Walcott's translucent slices. So far as one can judge from the illustrations, it is probable that what he took for limbs were really fragments of other trilobites. Such is certainly the case in his figures 9 and 10, where a number of more or less broken thoracic segments are present. The section of Encrinurus punctatus shown in figure 7 may possibly exhibit the position and folds of the ventral membrane beneath the axial lobe, and also, perhaps, the appendages. His figures 4, 5 and 8 show the hypostoma in section.
Pander (1830) described the hypostoma in greater detail than had been done by previous authors, but otherwise added nothing to the subject.
Sternberg (1830) thought he had individuals showing appendages, but judging from his poor figures, he was deceived by fragmentary specimens.
Green (1839 A, B, C) described specimens of Phacops from Berkeley Springs, West Virginia, which had the hypostoma in position, and appear to have had a tubular opening under the axial lobe. While appendages were not actually present, these specimens suggested fairly correct ideas about the swimming and breathing organs of trilobites. They were similar to the ones which Castelnau obtained, and all were perhaps from the same locality.
It is not worth while to do more than enumerate the other authors of this period: Hisinger 1837, Emmrich 1839, Milne-Edwards 1841, for they all shared the same views, and added nothing to what was already known.