such as Collies. It is the most common oral birth defect in horses. Class III malocclusion (MAL/3) Also called mesioclusion, prognathism, undershot jaw, and underbite, MAL/3 occurs when the upper teeth rest behind the lower equivalents. The maxilla is behind (maxillary retrognathism) and the mandible is forward (mandibular prognathism). It is more common in animals with brachycephalic skulls, such as pugs. This type of malocclusion is also often associated with rostral cross bite. Other malocclusions Level bite – end-to-end bite of the incisors. Genetically is a degree of prognathism. Wry mouth – refers to a variety of unilateral occlusal abnormalities. Genetically only affects one quadrant of the mandible or maxilla, such as one segment of the jaw being disproportionately sized relative to the other half. Oligodontia – only a few teeth are present Anodontia – congenital absence of teeth Hypodontia – one or a few teeth are missing Polydontia – presence of more teeth than is normal (supernumerary teeth) Retained deciduous (baby) teeth – occurs when erupting permanent teeth do not push deciduous teeth out. This is common in toy breed dogs. Oral lesions and masses Some animals will develop oral lesions, masses, or growths, which may be benign or malignant. These may be caused by tooth or gum infections, tumors, or genetic predisposition. Most pets do not show signs of oral masses until it has grown large enough to make chewing and swallowing difficult. Bad breath, excessive drooling, or bloody oral discharge may also be signs of an oral lesion or mass. Malignant tumors Melanoma – cancerous tumor that spreads to regional lymph nodes and lungs. Bone destruction is usually evident around the tumor. Most commonly observed in dogs; it is rare in cats. Squamous cell carcinoma – fast-growing tumor that is often ulcerated. It spreads slowly and invades bone tissue. It is the most common oral tumor in cats and second most common tumor in dogs. Fibrosarcoma – occurs at a younger age than other oral malignancies. It metastasizes slowly but is aggressively invasive. It is the second most common tumor in cats and third most common in dogs. Osteosarcoma – may affect the bones of the maxilla or mandible. Oral osteosarcoma is more responsive to surgical intervention than the appendicular version. Nonmalignant tumors Epulis – general term for any gingival mass. A biopsy and radiograph is required in order to differentiate the type. Peripheral odontogenic fibroma –