Cambridge's tuition in natural history at that time was not of a high standard, but Clark devised his own reading list and purchased specimens from local residents.
He followed up with notes on specimens from Cambridgeshire, Northamptonshire and at Whittlesea Mere, and in 1855 with 'Synonymic list of the British carnivorous water beetles, together with critical remarks and notices of foreign allied species' (The Zoologist, volume 13, pp. 4846–4869).
He collaborated with John Frederic Dawson on a revision of the classification and taxonomy of the British Carabidae.
A world catalogue of Phytophaga, a joint work with Henry Walter Bates, was unfinished at his death, only the first part having been published in 1866.
In December of the same year he accompanied Gray once again on a trip to Rio de Janeiro in Brazil.