For viruses, classification is normally based on type of nucleic acid, morphology, presence or absence of an envelope and replication properties. A number of families is recognized, with the family name ending in -viridae, . Poxviridae (double-stranded DNA , enveloped animal viruses). The viral genus has the suffix -virus, . Orthopoxvirus, and the viral species is given a common descriptive name ending with virus, . smallpox virus. In general, higher taxa . order, kingdom, have not yet been assigned, but viruses are usually separated into large groups based on host preference . animal viruses, plant viruses and bacterial viruses (see BACTERIOPHAGES ).