Which of the following is not a chondrichthyes?

Options

A. Scoliodon
B. Sting ray
C. Hippocampus
D. Torpedo

Explanation

To determine which of the following is not a Chondrichthyes, we need to understand what Chondrichthyes are and analyze each option provided.

### Step-by-Step Solution:

1. **Understand Chondrichthyes**:
- Chondrichthyes are a class of fish characterized by having a skeleton made of cartilage instead of bone. This group includes sharks, rays, and skates.

2. **Examine the Options**:
- We will look at each option provided to see if they belong to Chondrichthyes or not.

3. **Option 1: Skolydon**:
- Skolydon is a type of cartilaginous fish. Therefore, it belongs to Chondrichthyes. This option is not the answer.

4. **Option 2: Stingray**:
- Stingrays are also cartilaginous fish and belong to the Chondrichthyes class. This option is not the answer.

5. **Option 3: Hippocampus**:
- Hippocampus, commonly known as seahorses, are bony fish and belong to the class Osteichthyes. This option is the correct answer as it is not a Chondrichthyes.

6. **Option 4: Torpedo**:
- Torpedo is another type of cartilaginous fish, which means it belongs to Chondrichthyes. This option is not the answer.

7. **Conclusion**:
- After analyzing all the options, we conclude that the correct answer is **Hippocampus**, as it is not a Chondrichthyes.

### Final Answer:
**Hippocampus is not a Chondrichthyes.**